[clang] [RISCV][clang] Support XSfmm ABI attributes (PR #206260)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The names seem very similar to the Arm ones, but the semantics are subtly different.  I guess the biggest difference is the treatment of `__riscv_new`: there is no mechanism for spilling, it's just a hard error if you use a "new" function, or an unmarked function.

This is potentially a bit messy for the backend: it implies the compiler can't introduce calls as part of lowering.  Or maybe, the compiler can only introduce calls to `__riscv_preserves` functions.  But we don't really know what `memcpy` does internally, besides assuming it follows normal C ABI rules.

It also implies certain functions implicitly called by the runtime linker, like `_dl_runtime_resolve`, also preserve these registers, but I'm not sure if that's likely to lead to practical problems.

I'm not sure how we want to model this restriction.

https://github.com/llvm/llvm-project/pull/206260