NetBSD/vax 512MB support

[David Brownlee <abs%absd.org@localhost>]

I've run up test builds of netbsd-10, -9 and -8 with Ragge's patches
to support 512MB vaxen.

They are in the appropriate subdirs of https://sync.absd.org/vax/

If anyone would care to test (you can just grab a kernel, or the whole
distribution, as preferred), I'll get pullups sent so the official
NetBSD branches for -10, 9 and 8 will include the fixes

Thanks again to Ragge for the changes :)

David