Add preliminary support for cross building mips64 and armv6

This consider you have qemu-mips64 or qemu-arm installed in static version on your system
This is also considering you have Stacey Son patches and binmisctl on your system:
see: http://people.freebsd.org/~sson/imgact_binmisc/
and https://wiki.freebsd.org/QemuUserModeHowTo

This patch consider you condigure binmisc expecting qemu-mips64 or qemu-arm being launch from the following path:
/usr/bin/qemu-*