hashx: Fix compiled hash function on NetBSD

NetBSD includes the idea of a 'maximum protection' per-region,
and an mprotect which exceeds the max protection will be denied.

If we explicitly ask for a maximum which includes execute permission, we
can successfully swap our code buffer's permissions between read-write
and read-execute when each hash program is compiled.

With this patch, the crypto/hashx tests pass on NetBSD 9.
This addresses bug #40844