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

[Alex Bradbury via llvm-commits]

asb wrote:

> @asb I wonder if this is cause of the difference you were seeing between GNU objdump on GNU vs LLVM objects.

It's possible I made an error [when rebasing](https://gist.github.com/asb/68c249661bc68a3fa8f04d45a519874f), but unfortunately this doesn't seem to impact the issue I'm seeing. I'll file a separate bug report.

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