libgo: Merge from revision 18783:00cce3a34d7e of master library.

This revision was committed January 7, 2014.  The next
revision deleted runtime/mfinal.c.  That will be done in a
subsequent merge.

This merge changes type descriptors to add a zero field,
pointing to a zero value for that type.  This is implemented
as a common variable.

	* go-gcc.cc (Gcc_backend::implicit_variable): Add is_common and
	alignment parameters.  Permit init parameter to be NULL.

From-SVN: r211249

libgo/go/encoding/ascii85/ascii85_test.go

@@ -16,6 +16,11 @@ type testpair struct {
 }
 
 var pairs = []testpair{
+	// Encode returns 0 when len(src) is 0
+	{
+		"",
+		"",
+	},
 	// Wikipedia example
 	{
 		"Man is distinguished, not only by his reason, but by this singular passion from " +