Enable OpenMP build for 32-bit FreeBSD

Summary:
To be able to successfully build OpenMP on 32-bit FreeBSD, such as
FreeBSD/i386, I first had to provide a few wrappers (see D60916), and
then add `KMP_OS_FREEBSD` to the list of defines checked for 32-bit
architectures in `kmp_runtime.cpp`.

I have successfully built libomp.so and ran a bunch of test programs on
FreeBSD/i386 with this.

See also https://svnweb.freebsd.org/changeset/base/345283

Reviewers: emaste, jlpeyton, Hahnfeld

Reviewed By: jlpeyton

Subscribers: krytarowski, guansong, jdoerfert, openmp-commits, llvm-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D60917

llvm-svn: 359716