[lldb] [test] Fix TestGdbRemoteThreadName code on FreeBSD

Fix TestGdbRemoteThreadName to call ::pthread_setname_np instead
of ::pthread_set_name_np on FreeBSD.  While technically both names
are correct, the former is preferable because of compatibility
with Linux.  Furthermore, the latter requires `#include <pthread_np.h>`
that was missing causing the test to fail to compile.

Differential Revision: https://reviews.llvm.org/D90862