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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > The review context is on [reviews.llvm.org/D145214](https://reviews.llvm.org/D145214) . Moving it here just causes more confusion.
> > @alexfanqi is the author of the patch and they can drive a reland.
> 
> Can you elaborate what is confusing? The patch was already reviewed and accepted. I'm waiting on the author to reply back but I posted here for it to be tested. @alexfanqi asked me to land the patch last time ([reviews.llvm.org/D145214#4650928](https://reviews.llvm.org/D145214#4650928))

The patch is just a reland of https://reviews.llvm.org/D145214 . AIUI this pull request does not add anything on top of that, but the main author of commit 46cb8d9a325233ac11ed5e90367c43774294d87e is now you with a Co-authored-by: tag attributing Alex Fan. That's what you got when you posted the pull request and merged the pull request via the web UI, instead of letting Alex Fan post the patch here.

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