gcc: add support for gcc 5.1

gcc: add support for gcc 5.1

This commit adds support for gcc 5.1 in Buildroot. In terms of gcc
patches, compared to gcc 4.9.x:

 * Kept as is, sometimes after minor adjusments:

   100-uclibc-conf.patch
   301-missing-execinfo_h.patch
   810-arm-softfloat-libgcc.patch
   830-arm_unbreak_armv4t.patch
   840-microblaze-enable-dwarf-eh-support.patch
   850-libstdcxx-uclibc-c99.patch
   860-cilk-wchar.patch

 * Dropped:

   110-pr64896.patch
   111-pr65730.patch

 * Split in multiple parts:

   900-musl-support.patch

   The patches from Crosstool-NG for muls support are used instead of
   one single patch.

 * Renamed:

   910-gcc-poison-system-directories.patch to
   200-gcc-poison-system-directories.patch

   920-libgcc-remove-unistd-header.patch to
   201-libgcc-remove-unistd-header.patch

   Since the 9xx part of the series is now used by the various musl
   related patches.

We have tested the following configurations, with a minimal Busybox
system:

 * ARM, uClibc-ng
 * ARM, glibc
 * ARM, musl
 * x86, uClibc-ng and uClibc 0.9.33.2
 * x86, glibc
 * x86, musl

All of the configurations built fine. All the configurations boot fine
in Qemu, except x86/uClibc (either ng or 0.9.33.2), it segfaults when
running init:

devtmpfs: mounted
Freeing unused kernel memory: 300K (c1389000 - c13d4000)
init[1]: segfault at 0 ip b77708c1 sp bfa9bb0c error 4 in ld-uClibc-0.9.33.2.so[b776c000+6000]
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b

We'll give some time for the uClibc developers to fix the problem
before taking other measures in Buildroot to exclude gcc 5.1 from a
x86/uClibc configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>