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

[Piyou Chen via llvm-commits]

BeMg wrote:

Since this resolver function is expected to be available and interchangeable for both `libgcc` and `compiler-rt`, a formal specification for the resolver function interface is necessary. 

I've create one for this PR https://github.com/riscv-non-isa/riscv-c-api-doc/pull/74 and provide the three different candidate approach to achieve the same purpose.

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