ec: Add run time code selection for p521 field operations

ec: Add run time code selection for p521 field operations

This is only used if ECP_NISTP521_ASM is defined and this currently
only occurs on PPC64.

This simply chooses the C reference implementation, which will be the
default when custom code is available for certain CPUs.

Only the multiplication and squaring operations are handled, since the
upcoming assembly code only contains those.  This scheme can be easily
extended to handle reduction too.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Signed-off-by: Amitay Isaacs <amitay@ozlabs.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15401)