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

This patch allows using the name "rdma" as an alias for "rdm". The name
makes its way to target attributes as well as the command line via the
-march and -mcpu options. The motivation was originally to support this
in Function Multi Versioning but it also makes sense to align with GCC
on the command line.