llvm-*: Move all LLVM_BINUTILS symlinks to toolchain package

Some of the LLVM binary utilities were included in the Clang package
(because they did not set an explicit PACKAGE).

Add a new Makefile under clang/toolchain to create the symlinks and man
links for ar, c++filt, nm, and so on (without the llvm-* prefix) when
LLVM_BINUTILS is enabled (as it is by default).

PR: 293610
Reviewed by: bapt, ivy, brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55692