[x32] Add __ILP32__ macro for ILP32 platforms

Summary:
Add __ILP32__ and _ILP32 macro for corresponding platforms.
Cover x86_64-*-*-gnux32 with test.

Test Plan: test added

Reviewers: chandlerc, atanasyan

Subscribers: cfe-commits, dschuff, zinovy.nis

Differential Revision: http://reviews.llvm.org/D4473