Enable /usr/lib32 for o32 binaries on mips64.

Build and install an o32 set of libraries on mips64 suitable for
running o32 binaries via COMPAT_FREEBSD32. Enable COMPAT_FREEBSD32 in
MALTA64.

Reviewed by: jmallett, imp
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D9032