[llvm] [RISCV] Lower scalar `CLMUL` to `vclmul` if `clmul` instruction not available (PR #212616)

[Sean Clarke via llvm-commits]

xarkenz wrote:

@lukel97 or @topperc, would you mind merging for me unless we want to figure out what to do about the `noimplicitfloat` case? Thanks for the reviews!

https://github.com/llvm/llvm-project/pull/212616