Fix pthread_setname_np build error

Fix pthread_setname_np build error

My earlier patch to fix the pthread_setname_np build error on macOS
was incorrect.  While the macOS man page claims that
pthread_setname_np returns void, in <pthread.h> it is actually
declared returning "int".  I knew this earlier, but must have made
some mistake when preparing the patch for submission (perhaps when
removing the templates?).

This patch re-fixes the bug.  I'm also applying it to the 9.1 branch.

Tested by building on macOS High Sierra.

gdb/ChangeLog
2019-12-18  Tom Tromey  <tromey@adacore.com>

	PR build/25268:
	* gdbsupport/thread-pool.c (set_thread_name): Expect "int" return
	type on macOS.  Add comment.

Change-Id: Ib09da6ac33958a0d843f65df2a528112356e7de6