[TableGen][RISCV] Add initial support for marking profiles as experimental (#91993)

This is just the TableGen-side changes, split out as the minimal
testable unit. It doesn't yet transition RVA23 and friends to be
experimental (and add the necessary other changes for this to work).

Although choosing not to emit the SupportedExperimentalProfiles array if
no experimental profiles are present isn't consistent with what we do
for experimental extensions, we need to do this in order to avoid adding
a warning for the empty array when building LLVM for as long as we don't
have any experimental profiles defined.