system: also create a lib64 -> lib symlink on mips64n32

system: also create a lib64 -> lib symlink on mips64n32

In gcc 5.1.0, a change was introduced which causes internal search paths
inside the sysroot to be relative to 'lib64' rather than 'lib'. See [1] [2]
and [3].

For example for dtc:

     LD convert-dtsv0
    /opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/bin/ld: cannot find crt1.o: No such file or directory
    /opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/bin/ld: cannot find crti.o: No such file or directory
    collect2: error: ld returned 1 exit status
    make[1]: *** [Makefile:236: convert-dtsv0] Error 1
    make[1]: *** Waiting for unfinished jobs....
    make[1]: Leaving directory '/opt/buildroot/output/build/dtc-1.4.7'
    make: *** [package/pkg-generic.mk:241: /opt/buildroot/output/build/dtc-1.4.7/.stamp_built] Error 2

In this case, crt1.o was searched for in following locations:

    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/n32/octeon3/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/n32/octeon3/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/lib/mips64-octeon-linux-gnu/7.3.0/n32/octeon3/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/lib/../lib32-fp/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/lib64/mips64-octeon-linux-gnu/7.3.0/n32/octeon3/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/lib64/../lib32-fp/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/usr/lib64/mips64-octeon-linux-gnu/7.3.0/n32/octeon3/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/usr/lib64/../lib32-fp/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/lib/mips64-octeon-linux-gnu/7.3.0/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/opt/ext-toolchain/bin/../lib/gcc/mips64-octeon-linux-gnu/7.3.0/../../../../mips64-octeon-linux-gnu/lib/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/lib64/mips64-octeon-linux-gnu/7.3.0/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/lib64/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/usr/lib64/mips64-octeon-linux-gnu/7.3.0/crt1.o", R_OK) = -1 ENOENT (No such file or directory)
    16073 access("/opt/buildroot/output/host/mips64-buildroot-linux-gnu/sysroot/usr/lib64/crt1.o", R_OK) = -1 ENOENT (No such file or directory)

As can be seen above, all attempted paths contain 'lib64' as base,
instead of 'lib' or 'lib32', e.g.

.../sysroot/lib64/../lib32-fp/crt1.o
.../sysroot/lib64/crt1.o

This problem was detected on a gcc 7.x toolchain provided by Marvell as part
of their Octeon SDK. For this toolchain, here are the values of the paths
as detected by the Buildroot toolchain logic, for two different Octeon
processors:

- octeon2 (soft-float) (-mabi=n32 -march=octeon2):
SYSROOT_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/sys-root/;
ARCH_SYSROOT_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/sys-root/;
ARCH_SUBDIR=;
ARCH_LIB_DIR=lib32/octeon2;
SUPPORT_LIB_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/lib32/octeon2/

- octeon3 (hard-float) (-mabi=n32 -march=octeon3):
SYSROOT_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/sys-root/;
ARCH_SYSROOT_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/sys-root/;
ARCH_SUBDIR=;
ARCH_LIB_DIR=lib32-fp;
SUPPORT_LIB_DIR=/opt/buildroot/output/host/opt/ext-toolchain/mips64-octeon-linux-gnu/lib32-fp/

For both cases (MIPS64n32) Buildroot created a symlink 'lib32->lib', from
SYSTEM_LIB_SYMLINK in system/system.mk. Additionally, the function
create_lib_symlinks in
toolchain/toolchain-external/pkg-toolchain-external.mk will use ARCH_LIB_DIR
and create an additional link $(ARCH_LIB_DIR)->lib.
For the Octeon3 case this thus results in the following symlinks (where the
'lib32' one is normally not needed):

    lib32 -> lib/
    lib32-fp -> lib/

Since the toolchain is searching based on a 'lib64' component, it will fail
to find its internal paths.

To solve the problem, we need to create an additional symlink 'lib64':

    lib64 -> lib/

[1] https://github.com/gcc-mirror/gcc/commit/257ccd463a4f25495b252a3bdf989d057c54678d
[2] https://gcc.gnu.org/ml/gcc-patches/2014-10/msg03377.html
[3] https://gcc.gnu.org/ml/gcc-patches/2014-11/msg00539.html

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>