unix: add GPIO constants and structs

This adds everything needed to use Linux's GPIO V2 UAPI, including the
ioctls, structs and constants.
As on some architectures char is signed by default and the UAPI doesn't
make it unsigned explicitly a mkpost fixup is used to make the types
more usable (by things like unix.ByteSliceToString).

Change-Id: Icdb0ca4a225316ca5e948e21ab21789636297a57
Reviewed-on: https://go-review.googlesource.com/c/sys/+/752623
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>