The interpreter implementation of fctiwx was treating rounding
mode 0 as "round to nearest, ties towards zero", which is not
an actual IEEE-754 rounding mode. The IBM document mentioned
in a comment at the top of the function, on the other hand,
treats rounding mode 0 as "round to nearest, ties to even",
which makes more sense.
This fixes one of JMC's console-recorded F-Zero GX replays on
JitArm64. (JitArm64 uses an interpreter fallback for fctiwx.)