[clang] [llvm] [RISCV] Support for Zvabd fast-track proposal (PR #124239)

[Pengcheng Wang via cfe-commits]

wangpc-pp wrote:

Thanks for reviewing this PR! I will split it into several PRs and commit them separately.
For the record (updating):
1. https://github.com/llvm/llvm-project/pull/180139

https://github.com/llvm/llvm-project/pull/124239