Cleanup COMPAT_FREEBSD32 support.

This is a NOP.

The COMPAT_IA32 was renamed in r205014 to COMPAT_FREEBSD32 and
COMPAT_ARCH32 does not seem to have existed. Also remove some
leftovers from the sysent rework in r301404. Include
freebsd32_util.h for the freebsd32_sysent prototype.

X-MFC-With: r301404
Reported by: kib
MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division