cgo: add GoBytes, fix gmp example

Fixes #1640.
Fixes #2007.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4815063
diff --git a/misc/cgo/stdio/file.go b/misc/cgo/stdio/file.go
index 021cbf9..ab1e884 100644
--- a/misc/cgo/stdio/file.go
+++ b/misc/cgo/stdio/file.go
@@ -42,3 +42,4 @@
 }
 
 var Greeting = C.GoString(C.greeting)
+var Gbytes = C.GoBytes(unsafe.Pointer(C.greeting), C.int(len(Greeting)))