Disable tests that don't work when using gccgo.

--- a/go/buildutil/util_test.go
+++ b/go/buildutil/util_test.go
@@ -5,6 +5,7 @@
 // Incomplete source tree on Android.
 
 // +build !android
+// +build !gccgo
 
 package buildutil_test
 
--- a/go/gcimporter15/bexport_test.go
+++ b/go/gcimporter15/bexport_test.go
@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build go1.6
+// +build !gccgo
 
 package gcimporter_test
 
--- a/cmd/guru/guru_test.go
+++ b/cmd/guru/guru_test.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !gccgo
+
 package main_test
 
 // This file defines a test framework for guru queries.
--- a/cmd/stringer/endtoend_test.go
+++ b/cmd/stringer/endtoend_test.go
@@ -5,6 +5,7 @@
 // go command is not available on android
 
 // +build !android
+// +build !gccgo
 
 package main
 
--- a/cmd/stringer/golden_test.go
+++ b/cmd/stringer/golden_test.go
@@ -7,6 +7,8 @@
 // it provides a way to look at the generated code without having
 // to execute the print statements in one's head.
 
+// +build !gccgo
+
 package main
 
 import (
--- a/go/buildutil/allpackages_test.go
+++ b/go/buildutil/allpackages_test.go
@@ -5,6 +5,7 @@
 // Incomplete source tree on Android.
 
 // +build !android
+// +build !gccgo
 
 package buildutil_test
 
--- a/go/loader/loader_test.go
+++ b/go/loader/loader_test.go
@@ -15,6 +15,7 @@
 	"go/build"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -111,6 +112,9 @@
 }
 
 func TestLoad_MissingInitialPackage(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	var conf loader.Config
 	conf.Import("nosuchpkg")
 	conf.Import("errors")
@@ -129,6 +133,9 @@
 }
 
 func TestLoad_MissingInitialPackage_AllowErrors(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	var conf loader.Config
 	conf.AllowErrors = true
 	conf.Import("nosuchpkg")
@@ -253,6 +260,9 @@
 }
 
 func TestLoad_FromImports_Success(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	var conf loader.Config
 	conf.ImportWithTests("fmt")
 	conf.ImportWithTests("errors")
--- a/go/pointer/example_test.go
+++ b/go/pointer/example_test.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !gccgo
+
 package pointer_test
 
 import (
--- a/go/ssa/interp/interp_test.go
+++ b/go/ssa/interp/interp_test.go
@@ -5,6 +5,7 @@
 // +build go1.5
 
 // +build !android,!windows,!plan9
+// +build !gccgo
 
 package interp_test
 
--- a/go/ssa/ssautil/load_test.go
+++ b/go/ssa/ssautil/load_test.go
@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build go1.5
+// +build !gccgo
 
 package ssautil_test
 
--- a/refactor/eg/eg_test.go
+++ b/refactor/eg/eg_test.go
@@ -7,6 +7,7 @@
 // No testdata on Android.
 
 // +build !android
+// +build !gccgo
 
 package eg_test
 
--- a/refactor/importgraph/graph_test.go
+++ b/refactor/importgraph/graph_test.go
@@ -5,6 +5,7 @@
 // Incomplete std lib sources on Android.
 
 // +build !android
+// +build !gccgo
 
 package importgraph_test
 
--- a/refactor/rename/rename_test.go
+++ b/refactor/rename/rename_test.go
@@ -1127,6 +1127,9 @@
 }
 
 func TestDiff(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	if runtime.GOOS == "windows" {
 		t.Skipf("diff tool non-existent for windows on builders")
 	}
--- a/go/loader/example_test.go
+++ b/go/loader/example_test.go
@@ -4,6 +4,7 @@
 
 // +build go1.6
 // +build !windows
+// +build !gccgo
 
 package loader_test
 
--- a/go/loader/example15_test.go
+++ b/go/loader/example15_test.go
@@ -4,6 +4,7 @@
 
 // +build go1.5,!go1.6
 // +build !windows
+// +build !gccgo
 
 package loader_test
 
--- a/go/ssa/source_test.go
+++ b/go/ssa/source_test.go
@@ -28,6 +28,9 @@
 )
 
 func TestObjValueLookup(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	if runtime.GOOS == "android" {
 		t.Skipf("no testdata directory on %s", runtime.GOOS)
 	}
@@ -196,6 +199,9 @@
 // Ensure that, in debug mode, we can determine the ssa.Value
 // corresponding to every ast.Expr.
 func TestValueForExpr(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skip("Test disabled on gccgo.")
+	}
 	if runtime.GOOS == "android" {
 		t.Skipf("no testdata dir on %s", runtime.GOOS)
 	}
--- a/go/ssa/builder_test.go
+++ b/go/ssa/builder_test.go
@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build go1.5
+// +build !gccgo
 
 package ssa_test
 
--- a/go/ssa/example_test.go
+++ b/go/ssa/example_test.go
@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build go1.5
+// +build !gccgo
 
 package ssa_test
 
--- a/go/ssa/testmain_test.go
+++ b/go/ssa/testmain_test.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !gccgo
+
 package ssa_test
 
 // Tests of FindTests.  CreateTestMainPackage is tested via the interpreter.