Also copy doc comments from Go code to _cgo_export.h.

This is a step toward installing this generated file when using
-buildmode=c-archive or c-shared, so that C code can #include it.

Change-Id: I3a243f7b386b58ec5c5ddb9a246bb9f9eddc5fb8
Reviewed-on: https://go-review.googlesource.com/9790Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>