Fix a nasty miscompilation of 176.gcc on linux/x86 where we synthesized

a memset using 16-byte XMM stores, but where the stack realignment code
didn't work.  Until it does (PR2962) disable use of xmm regs in memcpy
and memset formation for linux and other targets with insufficiently
aligned stacks.

This is part of PR2888

llvm-svn: 58317