[AArch64][TargetParser] autogen ArchExtKind enum (#90314)

Thanks to ExtensionSet::toLLVMFeatureList, all values of ArchExtKind
should correspond to a particular -target-feature. The valid values of
-target-feature are in turn defined by SubtargetFeature defs.

Therefore we can generate ArchExtKind from the tablegen data. This is
done by adding an Extension class which derives from SubtargetFeature.

Because the Has* FieldNames do not always correspond to the AEK_
names ("extensions", as defined in TargetParser), and AEK_ names do
not always correspond to -march strings, some additional enum entries
have been added to remap the names. I have renamed these to make the
naming consistent, but split them into a separate PR to keep the diff
reasonable (#90320)