Makefile: Support universe-toolchain on non-FreeBSD

We currently pass MACHINE and MACHINE_ARCH as TARGET and TARGET_ARCH
respectively for universe-toolchain, but on non-FreeBSD these may not
have values that we understand (e.g. on Linux it will be x86_64 rather
than amd64) for TARGET/TARGET_ARCH (note that we do support them for
MACHINE/MACHINE_ARCH). Since the choice is a bit arbitrary and merely
determines what LLVM's default triple will be, use amd64 on non-FreeBSD
as a known-good default.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D41545