Use Clang and LLD as the default toolchain for MIPS

Now that we have updated the in-tree version of LLVM to 10.0, we have all the
necessary LLVM changes to use Clang+LLD as the default toolchain for MIPS.

Relnotes: yes
Reviewed By: emaste, jhb, brooks, kevans
Differential Revision: https://reviews.freebsd.org/D23204