internal/cpu: use #ifdef __s390x__ in C code

Patch by Jakub Jelinek.

Fixes https://gcc.gnu.org/PR89407

Change-Id: I985210bdd58940d0c082073d6fbc2c312632376a
Reviewed-on: https://go-review.googlesource.com/c/163297
Reviewed-by: Ian Lance Taylor <iant@golang.org>