llvm: fix some llvm-* tool links when WITH_LLVM_BINUTILS is set

llvm: fix some llvm-* tool links when WITH_LLVM_BINUTILS is set

A few Makefiles overrode LINKS and MLINKS when WITH_LLVM_BINUTILS was
set, which caused some llvm- prefixed tools to disappear.  One such case
was llvm-ranlib, which some ports invoke explicitly.

Use += when adding to LINKS and MLINKS under WITH_LLVM_BINUTILS.

PR:		270955, 270956, 270959
Submitted by:	jbeich
Reviewed by:	arichardson
Fixes: 021385aba562 ("Add WITH_LLVM_BINUTILS to install LLVM binuti...")
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39747