[clang] [llvm] [RISCV][LLVM][Clang] Add experimental `Zvvm` config intrinsics (PR #203774)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Is this the API we want?  This looks very similar to rounding modes (see https://reviews.llvm.org/D113439 / https://reviews.llvm.org/D121376 / etc.)

https://github.com/llvm/llvm-project/pull/203774