Another round of fixes for VEX version compatibility

YMM registers on x64 don't exist in old versions. VECRET and BBPTR
are new. needsBBP went away. x86 HWCAP checking is stricter, indicating
I'd misunderstood the comments about MMXEXT. Up to VEX r2797.
Fix zero_region sanity check for 32-bit OCaml

The old upper bound wrapped on 32-bit OCaml's 31-bit ints, causing it
to always fail even on sane values. Make the value a bit smaller
(512MB from 1024MB) while we're at it.