[llvm] [RISCV][TableGen] Get right experimental extension name (PR #90185)

[Alex Bradbury via llvm-commits]

asb wrote:

Alternatively, should we reject processor definitions that have experimental extensions? Are we sure that processors with experimental extensions interact properly with the gating of experimental extensions behind -menable-experimental-extensions for user facing tools like clang?

https://github.com/llvm/llvm-project/pull/90185