Fix DynamicLibraryTest.cpp on FreeBSD and NetBSD

Summary:

After rL301562, on FreeBSD the DynamicLibrary unittests fail, because
the test uses getMainExecutable("DynamicLibraryTests", Ptr), and since
the path does not contain any slashes, retrieving the main executable
will not work.

Reimplement getMainExecutable() for FreeBSD and NetBSD using sysctl(3),
which is more reliable than fiddling with relative or absolute paths.

Also add retrieval of the original argv[] from the GoogleTest framework,
to use as a fallback for other OSes.

Reviewers: emaste, marsupial, hans, krytarowski

Reviewed By: krytarowski

Subscribers: krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D33171

llvm-svn: 303015