Always pass -m to ld for converting binary files to kernel ELF objects

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.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7837