[SystemZ/zOS/GOFF] Restrict GOFF test to SystemZ target

The GOFF file format is only supported on SystemZ.

Author: redstar

llvm/test/MC/GOFF/empty-goff.s

@@ -1,5 +1,6 @@
 * RUN: llvm-mc <%s --triple s390x-ibm-zos --filetype=obj -o - | \
 * RUN:   od -Ax -tx1 -v | FileCheck --ignore-case %s
+* REQUIRES: systemz-registered-target
 
 * Header record:
 *  03 is prefix byte