lib/util: Delegate constant time memcmp to gnutls_memcmp()

lib/util: Delegate constant time memcmp to gnutls_memcmp()

gnutls_memcmp() is mostly identical to our own implementation, except
that ours will not break if supplied with 4 GiB or more of data.
However, using an external function permits us to disclaim
responsibility if some CPU/compiler combination happens to invalidate
our constant-time guarantee.

For reference, gnutls_memcmp() implementation:
https://gitlab.com/gnutls/gnutls/-/blob/78d9820de0d2eb2f8088e359779ee7342f5f089e/lib/safe-memfuncs.c#L41-67

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>