Use clang and lld as the default toolchain for RISCV.

• Enable clang and lld as system toolchains.
• Don't use external GCC for universe by default.
• Re-enable riscv64sf since it builds fine with clang + lld.

Reviewed by: emaste, mhorne
Relnotes: yes
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23089