test: disable a couple of precise GC tests for gccgo

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/41610043
diff --git a/test/deferfin.go b/test/deferfin.go
index f9a74eb..fa5a933 100644
--- a/test/deferfin.go
+++ b/test/deferfin.go
@@ -23,6 +23,10 @@
 	if runtime.GOARCH != "amd64" {
 		return
 	}
+	// Likewise for gccgo.
+	if runtime.Compiler == "gccgo" {
+		return
+	}
 	N := 10
 	count := int32(N)
 	var wg sync.WaitGroup