Restrict android-pix-x86-fyi-rel tasks to x64 only.

Make sure they don't get picked up by idle arm bots.

Tbr: bpastene@chromium.org,hypan@google.com
Bug: 961025,922145
Change-Id: Ica05585a90508354af6488b282955a9c59882a9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1734228Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683835}

testing/buildbot/waterfalls.pyl

@@ -627,6 +627,7 @@
         'mixins': [
           'pie-x86-emulator',
           'linux-xenial',
+          'x86-64',
         ],
         'os_type': 'android',
         'test_suites': {