llvm-objcopy: link man page as strip.1 when WITH_LLVM_BINUTILS is set

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39429

(cherry picked from commit 5cb402cc89c01e0a1f18f8c6a4c7d9cfc10d26ab)