Enable s390x_target_feature in core

This is needed for stdarch which now includes s390x target features.

Author: ehuss

library/core/src/lib.rs

@@ -211,6 +211,7 @@
 #![feature(powerpc_target_feature)]
 #![feature(riscv_target_feature)]
 #![feature(rtm_target_feature)]
+#![feature(s390x_target_feature)]
 #![feature(sha512_sm_x86)]
 #![feature(sse4a_target_feature)]
 #![feature(tbm_target_feature)]