Add support for cross-building for FreeBSD/mips and CheriBSD.
Add support for cross-building for FreeBSD/mips and CheriBSD.

For FreeBSD/mips set 'XMIPS=1' in the environment and build in
the obj.mips directory.

For CheriBSD, set 'WITH_CHERI=yes' in the environment and build
in the obj.mips64 directory.

bsdjhb/kdbg,bsdjhb/kdbg