MFC 304004, 304596, 304598, 304599, 304600, 304604, 304620, 304685, 304687,

304688, 304689, 304746, 304749, 304750, 304806, 305071, 305191:

Merge arm64 superpage support, however leave it disabled by default.

MFC after: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation