Update docs on correspondence between Go releases and GCC releases.

Update C type that corresponds to Go type `int`.

Drop out of date comments about Ubuntu and RTEMS.

Change-Id: Ic1b5ce9f242789af23ec3b7e7a64c9d257d6913e
Reviewed-on: https://go-review.googlesource.com/35631
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>