sys-boot/grub:0 - Properly fix misoptimization by -fcombine-stack-adjustmemts (gcc...

sys-boot/grub:0 - Properly fix misoptimization by -fcombine-stack-adjustmemts (gcc-4.9+)

GCC assumes that it can combine stacks from outer "grub_stage2()"
and inner trampoline "doit()" functions (optimisation -fcombine-stack-adjustments).

But doit() function clobbers %esp in inline assebly statement as:

  asm volatile ("movl %%esp, %0\n\tmovl %1, %%esp\n"
               : "=&r" (realstack) : "r" (simstack));

which tricks gcc into computing invalid local variable offsets from current %esp value.

Fix by hinting gcc about %esp change in clobber list.

Thanks to slyfox for his debugging and patch!  This properly fixes bug 564890, allowing
for the -O0 hack to be removed.

Package-Manager: portage-2.2.24