Use correct way to test for MIPS arch after rOMP355687

Summary:
I ran into some issues after rOMP355687, where __atomic_fetch_add was
being used incorrectly on x86, and this turns out to be caused by the
following added conditionals:

```
#if defined(KMP_ARCH_MIPS)
```

The problem is, these macros are always defined, and are either 0 or 1
depending on the architecture.  E.g. the correct way to test for MIPS
is:

```
#if KMP_ARCH_MIPS
```

Reviewers: petarj, jlpeyton, Hahnfeld, AndreyChurbanov

Reviewed By: petarj, AndreyChurbanov

Subscribers: AndreyChurbanov, sdardis, arichardson, atanasyan, jfb, jdoerfert, openmp-commits, llvm-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D60938

llvm-svn: 358911