llvm-objcopy: Install llvm-strip link

Just as elftoolchain's objcopy doubles as strip, so does LLVM's. 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.

(Note this is only a partial MFC, as WITH_LLVM_BINUTILS was not MFC'ed)

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)