cmd/cgo: use standard generated code comment

cmd/cgo: use standard generated code comment

Change cgo to follow https://golang.org/s/generatedcode.

For the C code we continue to use /* */ comments, so they don't follow
the format exactly. It doesn't really matter since the format is only
for Go code anyhow. This CL changes the C code to be similar for
consistency.

Fixes #25623

Change-Id: Idcfee53ec4069924d173ab8cedeb7bcfb7312863
Reviewed-on: https://go-review.googlesource.com/115035
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>