x86jit: Support fpu rounding modes for mul, etc.

Fixes Gods Eater Burst loading PSP savedata, but can no longer load old
savedata.
x86jit: Implement cvt.w.s.

Not really used that often, anyway, but easy enough and good for testing
that we set the rounding mode correctly.
armjit: Respect the rounding mode for mul/etc.
Add a frontend option for GEB saves.

May also affect performance, so can be disabled for that.
x86jit: Maintain the rounding mode always.

This should be less often than doing it per block that uses fpu, unless
the game doesn't use fpu much at all.
armjit: Maintain rounding mode throughout jit.
Merge pull request #6762 from unknownbrackets/fpu-rounding

Handle fpu rounding mode at least in jits