gcc: fix build of libsanitizer in gcc 4.9 and 5.x on PowerPC

gcc: fix build of libsanitizer in gcc 4.9 and 5.x on PowerPC

libsanitizer in gcc fails to build on PowerPC with gcc versions 4.9
and 5.x used in conjunction with glibc 2.25, with the following error:

../../../../gcc-host/libsanitizer/asan/asan_linux.cc: In function 'bool __asan::AsanInterceptsSignal(int)':
../../../../gcc-host/libsanitizer/asan/asan_linux.cc:222:20: error: 'SIGSEGV' was not declared in this scope
   return signum == SIGSEGV && common_flags()->handle_segv;

This commit adds a patch that has been submitted to upstream gcc
(https://patchwork.ozlabs.org/patch/725596/) but not merged. The patch
is no longer needed with gcc 6.x and later because the code has been
reworked.

Fixes Buildroot bug #10061

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Thomas: rework commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>