Export the sizeof(Ptr|Short|Int|Long|LongLong) consts. This allows users
to get this information (e.g. for alignment purposes) without using cgo
or generating these constants themselves.

Change-Id: I8640482bf67b89c2f2b6e9a116ba7bc268f8135a
Reviewed-on: https://go-review.googlesource.com/c/139617
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>