Generate TargetParser extension information from tablegen. This includes FMV extension information. FMV only extensions are represented by a separate tablegen class.
Use MArchName/ArchKindEnumSpelling to avoid renamings.
Cases where there is simply a case difference are handled by
consistently uppercasing the AEK_ name in the emitted code.
Remove some Extensions which were not needed.
These had AEK entries but were never actually used for anything.
They are not present in Extensions[] data.
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