Added support for Pi 4 GPIO, based on code the Raspberry Pi Foundation
published:

This check-in is based on a version of gpio-common.c.in posted to the PiDP-8 Google Group by Oscar Vermeulen, which is why we're giving him credit for this check-in, even though I have done quite a lot of changes to that code, mainly to match the existing code style.

This is a branch because it conflicts with the current tip-of-trunk and
because it's not yet tested on a Pi 4.

It is unknown whether this branch will end up overriding the method
we're using on trunk for Pi 0-3 or if we will instead wait for the
Foundation to publish a new libbcm_host that obviates some of what this
check-in does.