llvm-objcopy: Install llvm-strip, and optionally strip, links

Just as elftoolchain's objcopy doubles as strip, so does LLVM's. This
ensures that a strip binary is still present for WITH_LLVM_BINUTILS
builds. Note that we do not currently have a committed copy of the
manpage generated from the rST source so no manpage is installed for
(llvm-)strip.

Reported by: Shawn Webb <shawn.webb@hardenedbsd.org>
Tested by: Shawn Webb <shawn.webb@hardenedbsd.org>
MFC after: 1 week

(cherry picked from commit f28c1d0c5c6c6532df0dfa38eaf804343988d163)