[compiler-rt] [compiler-rt][RISCV] Implement __riscv_ifunc_select (PR #85790)

[Piyou Chen via llvm-commits]

BeMg wrote:

Base on https://github.com/riscv-non-isa/riscv-c-api-doc/pull/74. Update this patch with new abstract layer.

https://github.com/llvm/llvm-project/pull/85790