Use compiler driver to link BERI boot loaders

Do not hardcode elf64-tradbigmips as output format in BERI linker scrips.
Unfortunately, in-tree toolchain and external newer versions of binutils
mean two different things under that. When creating elf binaries using
external toolchain, gcc uses elf64-tradbigmips-freebsd and so linker
script file has to match in order for ld to be able to create the final loader
binary.

Rather than trying to guess, remove hardcoded output format directive from
the linker directive files and use CC to invoke the linker instead.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D9050