riscv: actually enable sanitizers
- Backport https://github.com/llvm/llvm-project/commit/b475ce39e8b1de3a70ea242473f136a567be46e3.
- Enable libclang_rt for riscv.
Previous commit missed it.
Reviewed by: dim
Differential Revision: https://reviews.freebsd.org/D34543
Approved by: re (gjb)
(cherry picked from commit 3781e77995e9b81afcbf7b3dc84b33b864c48794)
(cherry picked from commit e58a146c5d24172b53f18b953b3d29ee25bdb90c)