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)