runtime: don't build test files that use SetCgoTraceback for gccgo

The gofrontend doesn't support the runtime.SetCgoTraceback function,
    which is specifically for handling mixed Go and C tracebacks.
    Use a build tag to avoid compiling the runtime/testdata/testprogcgo
    files that refer to SetCgoTraceback. These files are not currently
    compiled anyhow, but they will be with a future gotools patch.
    
    Reviewed-on: https://go-review.googlesource.com/46452

From-SVN: r249556

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-bbc4f30a027ee09475d08aa4aeed1f2f7067e2e9
+e017833e4ce4a3bc3ba02eac4351b15c86157ec0
 
 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/pprof.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
 
 // Run a slow C function saving a CPU profile.

libgo/go/runtime/testdata/testprogcgo/raceprof.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build linux,amd64
+// +build !gccgo
 
 package main
 

libgo/go/runtime/testdata/testprogcgo/threadpprof.go

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build !plan9,!windows
+// +build !gccgo
 
 package main
 

libgo/go/runtime/testdata/testprogcgo/traceback.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
 
 // This program will crash.

libgo/go/runtime/testdata/testprogcgo/tracebackctxt.go

@@ -4,6 +4,8 @@
 
 // The __attribute__((weak)) used below doesn't seem to work on Windows.
 
+// +build !gccgo
+
 package main
 
 // Test the context argument to SetCgoTraceback.

libgo/go/runtime/testdata/testprogcgo/tracebackctxt_c.c

@@ -5,6 +5,8 @@
 // The C definitions for tracebackctxt.go. That file uses //export so
 // it can't put function definitions in the "C" import comment.
 
+// +build !gccgo
+
 #include <stdlib.h>
 #include <stdint.h>