runtime: disable memory profiler in gc_test

This lets the test pass on PPC64 GNU/Linux, which uses a much
larger page size and thus uses more memory to hold blocks
allocated for memory profiling.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6048054

src/pkg/runtime/gc_test.go

@@ -15,6 +15,8 @@ func TestGcSys(t *testing.T) {
 	runtime.ReadMemStats(memstats)
 	sys := memstats.Sys
 
+	runtime.MemProfileRate = 0 // disable profiler
+
 	itercount := 1000000
 	if testing.Short() {
 		itercount = 100000