cmd/cgo: accept __uint8_t as the uint8_t type

cmd/cgo: accept __uint8_t as the uint8_t type

This works around the NetBSD <stdint.h> which defines the type using
"#define" rather than typedef.

Fixes #30918
Updates #29878

Change-Id: I8998eba52139366ae46762bdad5fcae85f9b4027
Reviewed-on: https://go-review.googlesource.com/c/go/+/168337
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>