cmd/compile: embed type string header in rtype

Reduces binary size of cmd/go by 1%.

For #6853.

Change-Id: I6f2992a4dd3699db1b532ab08683e82741b9c2e4
Reviewed-on: https://go-review.googlesource.com/19692
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/type.go

@@ -24,7 +24,7 @@ type _type struct {
 	// If the KindGCProg bit is set in kind, gcdata is a GC program.
 	// Otherwise it is a ptrmask bitmap. See mbitmap.go for details.
 	gcdata  *byte
-	_string *string
+	_string string
 	x       *uncommontype
 	ptrto   *_type
 }