Handle the FS (flush-to-zero) bit in FCR31 for x86 JIT.
Don't unconditionally ClearRoundingMode() before setting it.
Fix broken rounding mode handling.
Revert to unconditional ClearRoundingMode() when setting FCR31.
Merge pull request #6828 from achurch/master

Handle the FS (flush-to-zero) bit in FCR31 for x86 JIT.