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

[Sam Elliott via llvm-commits]

lenary wrote:

This has stalled for more than a year, which is not good.

@joe-img can you rebase the PR.

My view is this is good enough for the moment. We can return to this later to emit fewer symbols in the case where a user uses `.option arch` to enable the same architecture in that's in the attributes (which, iiuc, doesn't need the ISA suffix on the symbol).

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

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