equix: Add NetBSD to "huge pages not supported" path

equix: Add NetBSD to "huge pages not supported" path

As suggested by @wiz on #40843, let's add an explicit check to
hashx_vm_alloc_huge() that avoids using a Linux-style default
on NetBSD targets.

This doesn't change the questionable Linux-style default,
but a future patch will disable this code by default so it's not a
portability liability.

(This code is in hashx's VM layer but it's actually only relevant to
equix.)

This addresses bug #40843. Another patch will disable huge pages
by default entirely, but this patch is sufficient to fix the NetBSD
build.