[libgc] Disable large stack clearing - it seems to be crashing,
particularly in combination with FMOD, and the comments indicate
that it's targeted at platforms we don't support (SPARC).
Case 564033.
Force stack alignment on OSX. Fixes crashes at libmono/libc boundary (vasprintf).
Case 547363.
Older clang doesn't support -mstackrealign for x86_64.