s390: enable -march=z196 flag

R=joransiu@ca.ibm.com, bjaideep@ca.ibm.com, machenbach@chromium.org, jkummerow@chromium.org, jochen@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2707103002
Cr-Commit-Position: refs/heads/master@{#43355}

gypfiles/toolchain.gypi

@@ -315,6 +315,8 @@
             'defines': [
               'V8_TARGET_ARCH_S390_LE_SIM',
             ],
+          }, {
+            'cflags': [ '-march=z196' ],
           }],
           ],
       }],  # s390