Prefix {TARGET,BUILD}_TRIPLE with LLVM_ to avoid Makefile.inc1 collision.

The Makefile.inc1 TARGET_TRIPLE is for specifying which -target is used
during the build of world.

MFC after: 2 weeks
Reviewed by: dim, imp
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12792