MFC r306041: Always pass -m to ld for converting binary files to ELF

This is in preparation for linking with LLVM's lld, which does not have
a compiled-in default output emulation. lld requires that it is
specified via the -m option, or obtained from the object file(s) being
linked.

This will also allow all build targets to share a common linker binary.

Sponsored by: The FreeBSD Foundation