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

This change is a preliminary step to support trampolines on RISC-V. Trampolines are used by flang to implement obtai

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

This change is a preliminary step to support trampolines on RISC-V. Trampolines are used by flang to implement obtaining the address of an internal program (i.e., a nested function in Fortran parlance).

In this change we lower `llvm.clear_cache` intrinsic on glibc targets to
`__riscv_flush_icache` which is what GCC is currently doing for Linux targets.

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