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

[emaste]

Summary

A few Makefiles overrode LINKS and MLINKS when WITH_LLVM_BINUTILS was
set, which caused e.g. the llvm-ranlib link to disappear.  Use += when
adding WITH_LLVM_BINUTILS LINKS and MLINKS.

Thanks jbeich for spotting the issue.

PR:             270955, 270956, 270959
Fixes: 021385aba562 ("Add WITH_LLVM_BINUTILS to install LLVM binuti...")
Sponsored by:   The FreeBSD Foundation

[emaste]

llvm-symbolizer had the same issue; nm and size did not have other LINKS, but do the same there for consistency