[AArch64] Complete the list of extensions supported by .arch and .arch_extension

This brings the list of extensions supported here up to date
with what is supported by current git versions of binutils.

Also add a comment to AArch64TargetParser to remind people to
consider adding new ones to the list supported in assembly.

In the case of the "rdma" extension, there's a slight surprise:
LLVM knows of the extension under the name "rdm", while binutils
has it named "rdma". However, binutils appears to accept any
abbreviated prefix of an arch extension, so it does accept the
form "rdm" too even if it formally considers it called "rdma".

Support both spellings for the extensions here, for simplicity.

Differential Revision: https://reviews.llvm.org/D151981