toolchain-external: handle ld.so fixups centrally

toolchain-external: handle ld.so fixups centrally

Normally, the Buildroot toolchain logic copies all required libraries from
the external toolchain to the staging directory, including the dynamic
loader ld-*.so.
There are cases, however, where the dynamic loader is _not_ automatically
copied to staging. This happens when the dynamic loader is not inside
ARCH_LIB_DIR itself (e.g. lib64), but instead resides in 'lib' (assume, of
course, that ARCH_LIB_DIR != 'lib').

Currently, this is fixed in a toolchain-specific fixup, e.g. by recreating a
missing symlink or copying over a missing file. Such toolchain specific
fixups are not very nice.

Moreover, in a subsequent patch, the value of ARCH_LIB_DIR changes for some
toolchains, causing them to have the same problem of a missing dynamic
loader. This used to be the case for older Linaro toolchains with libraries
in 'lib/<tuple>': Buildroot used to set ARCH_LIB_DIR=lib but the mentioned
patch changes it to 'lib/<tuple>' instead. As a result, the files directly
under 'lib/' will no longer be copied. There should be none, but the dynamic
loader is a notable exception.
[Note: support for these older Linaro toolchain has been removed in 2016.11]

Instead, copy over the ld.so file(s)/link(s) from the extracted toolchain
into staging, in the central copy_toolchain_sysroot function. The existing
toolchain logic will then handle the copy of these files from staging to
target.
This means the toolchain-specific fixups can be removed.

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