cmd/cgo: roll back "use C exact-width integer types to represent Go types"

Roll back CL 159258 and CL 168337. Those changes broke existing
code. I can't see any way to keep existing code working while also
producing good error messages for types like C.ulong (such as the ones
already tested for in misc/cgo/errors).

This is not an exact roll back because parts of the code have changed
since those CLs.

Updates #29878
Fixes #31093

Change-Id: I56fe76c167ff0ab381ed273b9ca4b952402e1434
Reviewed-on: https://go-review.googlesource.com/c/go/+/180357


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>