Revert "Avoid using TARGET_ARCH in llvm.build.mk"

This reverts commit 8534e6be8110a8126268a38dc0557a2d15615ce9, and adds
a cautionary note that there are dragons about that should be considered
when changing it.

PR: 267026
Reviewed by: dim, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D36981

(cherry picked from commit ed7910fbac07ee39d10528994e49ada2d0f2d754)