Fix interoperability test compilation on FreeBSD

Fix interoperability test compilation on FreeBSD

Summary:
While building the 8.0 releases on FreeBSD, I encountered the following
error in the regression tests, where ompt/misc/interoperability.cpp
failed to compile, with:

```
projects/openmp/runtime/test/ompt/misc/interoperability.cpp:7:10: fatal error: 'alloca.h' file not found
#include <alloca.h>
         ^~~~~~~~~~
```

Like on NetBSD, alloca(3) is defined in <stdlib.h> instead.

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

Reviewed By: jlpeyton

Subscribers: jdoerfert, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D59736

llvm-svn: 356936