runtime: lock M during cgo call

cgo should lock the M.
    
    
    See also https://golang.org/cl/18882 .
    
    Reviewed-on: https://go-review.googlesource.com/18883

From-SVN: r233670

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-1c3747d20789c73447ff71cbc739f7423c4bdf67
+156f5f0152797ac2afe5f23803aeb3c7b8f8418e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-cgo.c

@@ -41,6 +41,8 @@ syscall_cgocall ()
   if (runtime_needextram && runtime_cas (&runtime_needextram, 1, 0))
     runtime_newextram ();
 
+  runtime_lockOSThread();
+
   m = runtime_m ();
   ++m->ncgocall;
   g = runtime_g ();
@@ -70,6 +72,8 @@ syscall_cgocalldone ()
      _cgo_panic will already have exited syscall mode.  */
   if (g->status == Gsyscall)
     runtime_exitsyscall ();
+
+  runtime_unlockOSThread();
 }
 
 /* Call back from C/C++ code to Go code.  */