Fix non-FreeBSD host lib32 build for TARGET=amd64

When building on non-FreeBSD systems we need to pass an explicit target
triple to clang otherwise it will attempt to build with the host triple.
This also has advantages when building on a FreeBSD host: we now tell
clang that we are targeting at least FreeBSD 12.0 instead of an older
version so it can enable newer features.

Reviewed By: brooks (mentor)
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D16842