[X86] Use FILD/FIST to implement i64 atomic load on 32-bit targets with X87, but no SSE2

If we have X87, but not SSE2 we can atomicaly load an i64 value into the significand of an 80-bit extended precision x87 register using fild. We can then use a fist instruction to convert it back to an i64 integer and store it to a stack temporary. From there we can do two 32-bit loads to get the value into integer registers without worrying about atomicness.

This matches what gcc and icc do for this case and removes an existing FIXME.

Differential Revision: https://reviews.llvm.org/D60156

llvm-svn: 358211