mesa: use build flag to ensure stack is realigned on x86

mesa: use build flag to ensure stack is realigned on x86

Nowadays GCC assumes stack pointer is 16-byte aligned even on 32-bits, but that is an assumption OpenGL drivers (or any dynamic library for that matter) can't afford to make as there are many closed- and open- source application binaries out there that only assume 4-byte stack alignment.

V4: fix comment and indentation

V3: move all sse4.1 build flag config to the same location
 and add comment as to why we need to do the realign

V2: use $target_cpu rather than $host_cpu
  and setup build flags in config rather than makefile

https://bugs.freedesktop.org/show_bug.cgi?id=86788
Signed-off-by: Timothy Arceri <t_arceri@yahoo.com.au>
Reviewed-by: Matt Turner <mattst88@gmail.com>
CC: "10.4" <mesa-stable@lists.freedesktop.org>