s390x: Fix build on s390x with 'disable-asm'

Do not define S390X_MOD_EXP for a NO_ASM build, this would result in
unresolved externals for s390x_mod_exp and s390x_crt.

Signed-off-by: Ingo Franzki <ifranzki@linux.ibm.com>

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23458)

include/crypto/bn.h

@@ -116,7 +116,8 @@ OSSL_LIB_CTX *ossl_bn_get_libctx(BN_CTX *ctx);
 
 extern const BIGNUM ossl_bn_inv_sqrt_2;
 
-#if defined(OPENSSL_SYS_LINUX) && !defined(FIPS_MODULE) && defined (__s390x__)
+#if defined(OPENSSL_SYS_LINUX) && !defined(FIPS_MODULE) && defined (__s390x__) \
+    && !defined (OPENSSL_NO_ASM)
 # define S390X_MOD_EXP
 #endif