[x32] Add __int128 support

Summary:
Adding __int128 support explicitly for x86_64 because currently it's on
only when pointer size >= 64 which is not the case for x32.

Test Plan: One of the tests using __int128 is updated

Reviewers: atanasyan, chandlerc

Subscribers: cfe-commits, rob.khasanov, zinovy.nis, dschuff

Differential Revision: http://reviews.llvm.org/D4755