cmd/cgo: write a string rather than building an AST

This generates the same code as before, but does so directly rather
than building an AST and printing that. This is in preparation for
later changes.

Change-Id: Ifec141120bcc74847f0bff8d3d47306bfe69b454
Reviewed-on: https://go-review.googlesource.com/c/142883


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>