Fix gettid warnings on FreeBSD

Summary:
[Split off from D59451 to get this fix in separately]

While building the 8.0 releases on FreeBSD, I encountered the following
warnings in openmp quite a few times:

```
In file included from projects/openmp/runtime/src/kmp_settings.cpp:27:
projects/openmp/runtime/src/kmp_wrapper_getpid.h:35:2: warning: #warning is a language extension [-Wpedantic]
#warning No gettid found, use getpid instead
 ^
projects/openmp/runtime/src/kmp_wrapper_getpid.h:35:2: warning: No gettid found, use getpid instead [-W#warnings]
2 warnings generated.
```

I added a gettid wrapper that uses FreeBSD's pthread_getthreadid_np(3)
function for this.

Reviewers: emaste, jlpeyton, krytarowski, mgorny, protze.joachim

Reviewed By: jlpeyton

Subscribers: jfb, jdoerfert, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D59735

llvm-svn: 356934