[llvm] [RISCV][MC] Implement ISA mapping symbols (PR #67541)

[Joe Faulls via llvm-commits]

joe-img wrote:

> 
> For the testDefault3, maybe $x.3 is enough. But on the other hand, GCC always emits mapping symbols for all functions so this way should be fine.

I see what you mean. I agree, I think $x.3 should be enough: `an instruction mapping symbol without ISA string means using ISA configuration from ELF attribute` from the [psabi doc](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#mapping-symbol). However, I see no harm in explicitly having the ISA string on the mapping symbol. It could even make things a little clearer.


https://github.com/llvm/llvm-project/pull/67541