[xctool] Update xctool and use x86_64

Summary:
This newer version of xctool (which is actually used in the cassowary
upstream) includes some fixes for xcode6. Additionally, since xcode6 only has
64-bit simulators afaik, we need to look for build output in a different directory.

Test Plan:
Run `arc unit` successfully in a containing project, without
'NSInvalidArgumentException' or a failure to find build files.

Reviewers: bobo

Reviewed By: bobo

Differential Revision: https://phabricator.robinhood.com/D1869