runtime: Use atomic load for in entersyscall.

    
    Reportedly fixes PPC64 deadlock.
    
    From a comment by Gabriel Russell.
    
    Fixes golang/go#15051.
    
    Reviewed-on: https://go-review.googlesource.com/21450

From-SVN: r234694

gcc/go/gofrontend/MERGE

-3de822d11255d439fac9717897b017aae2de18c2
+8edf085a94579bd819a10f50328233812ceeb950
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/proc.c

@@ -2042,7 +2042,7 @@ doentersyscall()
 	m->mcache = nil;
 	m->p->m = nil;
 	runtime_atomicstore(&m->p->status, Psyscall);
-	if(runtime_sched.gcwaiting) {
+	if(runtime_atomicload(&runtime_sched.gcwaiting)) {
 		runtime_lock(&runtime_sched);
 		if (runtime_sched.stopwait > 0 && runtime_cas(&m->p->status, Psyscall, Pgcstop)) {
 			if(--runtime_sched.stopwait == 0)