FPU: Implement floating convert to integer instructions

FPU: Implement floating convert to integer instructions

This implements fctiw, fctiwz, fctiwu, fctiwuz, fctid, fctidz, fctidu
and fctiduz, and adds tests for them.

There are some subtleties around the setting of the inexact (XX) and
invalid conversion (VXCVI) flags in the FPSCR.  If the rounded value
ends up being out of range, we need to set VXCVI and not XX.  For a
conversion to unsigned word or doubleword of a negative value that
rounds to zero, we need to set XX and not VXCVI.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>