Bugs in gcc 5.3.0, 5.4.0 with force_align_arg_pointer

[Daniel Santos]

Just to let you guys know, there's a bug in gcc 5.3 & 5.4 that causes 
wine's 64 bit code to be generated as if the incoming stack boundary was 
8 bytes. This makes the pro/epilogues all bloaty with unaligned movups 
for SSE saves/restores. This can be avoided by actually *not* using 
force_align_arg_pointer or -mstackrealign. Note however that when using 
either of those, we should supply -mincoming-stack-boundary=4 so that 
gcc knows what alignment to force the arg pointer to... except that that 
is also expertly broken.

I'm not aware of this actually causing any crashes or UB, just producing 
slower, bloaty code. There was also PR69140 that caused build breakage 
when the stack pointer wasn't properly aligned, but I guess this is a 
flaw that got exposed once force_align_arg_pointer started breaking 
alignment on x64 abi.

Daniel