[clang] [llvm] [RISCV] Allow `Zicsr`/`Zifencei` to duplicate with `g` (PR #136842)

[Ana Pazos via llvm-commits]

apazos wrote:

This matter was addressed at the RVI Toolchain SIG meeting on 05/08/25.

It was concluded that the duplication of implied extensions, such as in the cases of G and Zifencei, and F and Zicsr, can be silently disregarded. Both GCC and LLVM toolchains are consistent with this approach through the application of this LLVM patch. 

However, the duplication of unrelated extensions should result in an error message indicating an invalid duplicated extension, which is already implemented in both toolchains.


https://github.com/llvm/llvm-project/pull/136842