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 2a53df6947)

crypto/modes/gcm128.c

@@ -485,7 +485,11 @@ static void gcm_get_funcs(struct gcm_funcs_st *ctx)
 #elif defined(GHASH_ASM_ARM)
     /* ARM defaults */
     ctx->gmult = gcm_gmult_4bit;
+# if !defined(OPENSSL_SMALL_FOOTPRINT)
     ctx->ghash = gcm_ghash_4bit;
+# else
+    ctx->ghash = NULL;
+# endif
 # ifdef PMULL_CAPABLE
     if (PMULL_CAPABLE) {
         ctx->ginit = (gcm_init_fn)gcm_init_v8;