Set the rounding mode for the X86 FPU to 64-bits instead of 80-bits. We

don't support long double anyway, and this gives us FP results closer to
other targets.

This also speeds up 179.art from 41.4s to 18.32s, by eliminating a problem
with extra precision that causes an FP == comparison to fail (leading to
extra loop iterations).

llvm-svn: 18895