[mips] Correct size_t and ptrdiff_t for N32.

Summary:
Correct size_t to be unsigned int and ptrdiff_t to be signed long. The types were the correct size before this change but
the exact type matters for name mangling and exception handling in C++.

Reviewers: atanasyan

Reviewed By: atanasyan

Differential Revision: http://reviews.llvm.org/D3470