Commit b946bedd09d3bd1 ("Previous versions of bsd.own.mk [...]")
mentions that bsd.own.mk included bsd.compiler.mk as a temporary
workaround and was destined to be removed in FreeBSD 12. Do that now.
Also, update some Makefiles, which need to include bsd.compiler.mk for
buildkernel and buildworld to pass.
PR: 203540
Fixes: 0bebad8d072b bsd.own.mk: Deorbit compat include of bsd.compiler.mk
Fixes: 73f37a69f65e Revert "bsd.own.mk: Deorbit compat include of bsd.compiler.mk"