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

[Andreu Carminati via llvm-commits]

andcarminati wrote:

I have one suggestion, considering https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#mapping-symbol, the following paragraph:

>The mapping symbol for instruction($x) indicates the start of a sequence of instructions. and it has an optional ISA string, which means the following code regions are using ISA is different than the ISA recorded in the arch attribute.

Can we emit just $x.<any> when some function uses the same features as in arch attribute?

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