Always pass the linker emulation via -m when linking modules and kernels.

Previously the linker emulation was only passed when building binary
objects for firmware modules. This change always passes the desired
output format for kernel modules and kernels rather than requiring the
toolchain's default output format to match the desired output format.
This in turn permits use of external toolchains whose default output
format does not match the desired output format.

Reviewed by: imp, emaste
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D10085