V8 Buildbot: Move swarming staging tester to 64 bits slave.
V8 Buildbot: Move swarming staging tester to 64 bits slave.

Need real 64 bit slaves as now linux 64 is in staging.

TBR=kjellander@chromium.org,tandrii@chromium.org
BUG=chromium:535160

Review URL: https://codereview.chromium.org/1406963006

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@297304 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build