riscv: actually enable sanitizers
- Backport https://github.com/llvm/llvm-project/commit/b475ce39e8b1de3a70ea242473f136a567be46e3.
- Enable libclang_rt for riscv.
Previous commit missed it.
MFC after: 3 days
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D34543
(cherry picked from commit 3781e77995e9b81afcbf7b3dc84b33b864c48794)