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

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18895 91177308-0d34-0410-b5e6-96231b3b80d8