x64FPURoundMode: use fesetround() instead of asm
x64FPURoundMode: always set x87 precision

Set the x87 precision, even on x64. Since we are using x87 instructions
in the JIT now, we can't guarantee that x87 precision will never
influence Dolphin on x64.
Merge pull request #329 from Tilka/rounding

Small FPU settings changes