[clang] [compiler-rt] [RISCV][compiler-rt] Update __init_riscv_feature_bits prototype (PR #101472)

[Piyou Chen via cfe-commits]

BeMg wrote:

> Alongside #101449, this seems to be extending the interface beyond what is described in the [current draft spec](https://github.com/riscv-non-isa/riscv-c-api-doc/pull/74). Is there some other source of truth I should be looking for? Is this a proposal to change the spec, or an implementation of something that was already decided (at least tentatively)?

I will soon update these changes in https://github.com/riscv-non-isa/riscv-c-api-doc/pull/74. These changes are based on discussions with @kito-cheng and @preames.

https://github.com/llvm/llvm-project/pull/101472