[llvm] [RISCV] Lower llvm.clear_cache to __riscv_flush_icache for glibc targets (PR #93481)

[Kito Cheng via llvm-commits]

kito-cheng wrote:

I agree that should just gate with linux rather than glibc, but I am incline do not including OS specific stuff within psABI if possible (although we already has document `wchat_t` in psABI)

https://github.com/llvm/llvm-project/pull/93481