[clang] [TSAN] add support for riscv64 (PR #68735)

[via cfe-commits]

hiraditya wrote:

Ah that's unfortunate.; sorry about that. In my local copy of git log (before merging)

```
commit 5b85613483861b3734edf9c94be390f2b3b0dd2a (HEAD -> riscv_tsan)
Author: Alex Fan <alex.fan.q at gmail.com>
Date:   Fri Oct 6 12:32:38 2023 -0700
```

so i thought github would respect that.

https://github.com/llvm/llvm-project/pull/68735