Use ARCH instead of MACHINE to check for mips.

Cross-building ports via CROSS_TOOLCHAIN and CROSS_SYSROOT only set
ARCH to the target arch. MACHINE still references the host. This
unbreaks cross-building ports for mips.

Reviewed by: jbeich, portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D22592