runtime/pprof: fix test

gogo called from GC is okay
for the same reasons that
gogo called from System or ExternalCode is okay.
All three are fake stack traces.

Fixes #8408.

LGTM=dvyukov, r
R=r, dvyukov
CC=golang-codereviews
https://golang.org/cl/152580043

src/runtime/pprof/pprof_test.go

@@ -249,7 +249,7 @@ func TestGoroutineSwitch(t *testing.T) {
 			// exists to record a PC without a traceback. Those are okay.
 			if len(stk) == 2 {
 				f := runtime.FuncForPC(stk[1])
-				if f != nil && (f.Name() == "System" || f.Name() == "ExternalCode") {
+				if f != nil && (f.Name() == "System" || f.Name() == "ExternalCode" || f.Name() == "GC") {
 					return
 				}
 			}