[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207093 91177308-0d34-0410-b5e6-96231b3b80d8