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.
    
    Reviewed-on: https://go-review.googlesource.com/46451

From-SVN: r249555

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-f70ef19badafb85b1caa72b51b0082deb48d433a
+bbc4f30a027ee09475d08aa4aeed1f2f7067e2e9
 
 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/callback.go

@@ -7,6 +7,9 @@
 package main
 
 /*
+#cgo CFLAGS: -pthread
+#cgo LDFLAGS: -pthread
+
 #include <pthread.h>
 
 void go_callback();