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

[Kito Cheng via cfe-commits]

https://github.com/kito-cheng commented:

Need negative test for __riscv_* with unsupported string, e.g. `__riscv_new("#!$@")`

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