fix small footprint builds on arm

Building with '-D OPENSSL_SMALL_FOOTPRINT' for aarch64 fails due to
'gcm_ghash_4bit' being undeclared.  Fix that by not setting the function
pointer when building with OPENSSL_SMALL_FOOTPRINT, matching openssl
behavior on x86.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25419)

(cherry picked from commit 2a53df69)