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

(cherry picked from commit 41df1d60e3237df2106dddf357e3205bf29f85d5)