[clang] [llvm] [TargetParser][AArch64] Add alias for FEAT_RDM. (PR #80540)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

FYI, see 4b8d9abca7d0280878fb12de331e688ee85d7cd8 for another existing case where we already support both `rdm` and `rdma`. But I don't think that case can share any of the aliasing logic from here.

https://github.com/llvm/llvm-project/pull/80540