[dev.cc] cmd/cgo: generate only Go source files

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

We changed cgo to write the actual function wrappers in Go
for Go 1.4. The only code left in C output files was the definitions
for pointers to C data and the #pragma cgo directives.
Write both of those to Go outputs instead, using the new
compiler directives introduced in CL 169360043.

(Still generating C files in gccgo mode.)

LGTM=r
R=r
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/169330045

src/cmd/cgo/main.go

@@ -147,6 +147,7 @@ var fset = token.NewFileSet()
 
 var dynobj = flag.String("dynimport", "", "if non-empty, print dynamic import data for that file")
 var dynout = flag.String("dynout", "", "write -dynobj output to this file")
+var dynpackage = flag.String("dynpackage", "main", "set Go package for dynobj output")
 var dynlinker = flag.Bool("dynlinker", false, "record dynamic linker information in dynimport mode")
 
 // These flags are for bootstrapping a new Go implementation,