gcc: fix build of gcc 6.x on Microblaze

Due to patch 840-microblaze-enable-dwarf-eh-support.patch, gcc 6.x
does not build:

../../gcc/config/microblaze/microblaze.c: In function 'void microblaze_expand_epilogue()':
../../gcc/config/microblaze/microblaze.c:3046: error: 'gen_rtx_raw_REG' was not declared in this scope

This patch was originally added to gcc 4.9 to make it capable of
building glibc. However, this is no longer needed with gcc 6.x, which
builds glibc just fine.

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