[llvm] [RISCV] Add support for new unprivileged extensions defined in profiles spec (PR #77458)

[Brandon Wu via llvm-commits]

4vtomat wrote:

Can you also add the test cases in:
1. `clang/test/Preprocessor/riscv-target-features.c`
2.` llvm/test/MC/RISCV/attribute-arch.s`

https://github.com/llvm/llvm-project/pull/77458