Fix CC being wrong during install* targets.

Move CROSS_TOOLS stuff to top of file (before bsd.compiler.mk) so that
decisions made by bsd.compiler.mk can properly affect the defaults in
src.opts.mk. Move that to after bsd.compiler.mk. Add a comment about
why we include bsd.compiler.mk here despite the fact that src.opts.mk
currently does too. Also remove bsd.arch.inc.mk that's been OBE.

Differential Revision: https://reviews.freebsd.org/D4087