I've committed this patch, pre-approved by Thomas. It's basically just a 
search and replace.

I had originally invented acc_device_gcn for the device number, but as 
Thomas pointed out the OpenACC documentation already uses 
acc_device_radeon for AMD devices. Presumably this is what any existing 
code will use, if anything, so we ought to be compatible.

There's no official release using the "wrong" name, so I don't believe 
we need to retain that name for any reason.

I've tested that there are no regressions.

Andrew