runtime: allocate more stack space in CgoCallbackGC test

    
    Allocate enough stack space so that the test will work on a system
    that does not support split stacks.
    
    This test is actually not very meaningful for gccgo at present, but it
    doesn't hurt to keep running it.
    
    Updates golang/go#20931
    
    Reviewed-on: https://go-review.googlesource.com/50630

From-SVN: r250433

gcc/go/gofrontend/MERGE

-6572f7e35f962bdb8a7c174920dbb70350b96874
+e34cb8dee6c1f215329e0eea79202b48cb83817c
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/testdata/testprogcgo/callback.go

@@ -23,7 +23,9 @@ static void foo() {
     pthread_t th;
     pthread_attr_t attr;
     pthread_attr_init(&attr);
-    pthread_attr_setstacksize(&attr, 256 << 10);
+    // For gccgo use a stack size large enough for all the callbacks,
+    // in case we are on a platform that does not support -fsplit-stack.
+    pthread_attr_setstacksize(&attr, 512 * 10000);
     pthread_create(&th, &attr, thr, 0);
     pthread_join(th, 0);
 }