libthr: Force the thr_wake() symbol to be resolved during initialization

Otherwise the lock upgrade performed by rtld's load_filtees() can result
in infinite recursion, wherein:
1. _rtld_bind() acquires the bind read lock,
2. the source DSO's filtees haven't been loaded yet, so the lock upgrade
   in load_filtees() cause rtld to jump to _rtld_bind() and release the
   bind lock,
3. _thr_rtld_lock_release() calls _thr_ast(), which calls thr_wake(),
   which hasn't been resolved yet,
4. _rtld_bind() acquires the bind read lock in order to resolve
   thr_wake(),
5. ...

See the linked pull request for an instance of this problem arising with
libsys.  That particular instance is also worked around by commit
e7951d0b04e6.

Reported by:	brooks
Reviewed by:	kib
Pull Request:	https://github.com/freebsd/freebsd-src/pull/908
MFC after:	1 week
Sponsored by:	Innovate UK

(cherry picked from commit 36f0a34ca645d49ec79d60ea7e773374ef0991ea)

lib/libthr/thread/thr_rtld.c

@@ -238,6 +238,7 @@ _thr_rtld_init(void)
 
 	mprotect(NULL, 0, 0);
 	_rtld_get_stack_prot();
+	thr_wake(-1);
 
 	li.rtli_version = RTLI_VERSION;
 	li.lock_create  = _thr_rtld_lock_create;