runtime: build testprogcgo with -pthread

Building this test with gccgo requires an explicit -pthread option to
be passed to the C compiler, so that it links against -lpthread.
This test is not built today, but it will be soon with a future patch.

Change-Id: I678bad1f486aba206aed44f91e9bf5c9499e2e4c
Reviewed-on: https://go-review.googlesource.com/46451
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/libgo/go/runtime/testdata/testprogcgo/callback.go b/libgo/go/runtime/testdata/testprogcgo/callback.go
index 7d9d68d..7b58f4a 100644
--- a/libgo/go/runtime/testdata/testprogcgo/callback.go
+++ b/libgo/go/runtime/testdata/testprogcgo/callback.go
@@ -7,6 +7,9 @@
 package main
 
 /*
+#cgo CFLAGS: -pthread
+#cgo LDFLAGS: -pthread
+
 #include <pthread.h>
 
 void go_callback();