The GCN port outputs stdout and stderr via a shared-memory interface.
Previously the buffer was limited to 1000 write operations, which was enough
for testing purposes, but easy to exhaust.
This patch implements a new circular buffering system allowing a greater
amount of output. The interface must allow hundreds of hardware threads to
output simultaneously. The new limit is UINT32_MAX write operations.
Unfortunately, there's no way to tell if the host side has also been updated.
This code will misbehave unless the gcn-run from GCC is also updated (although
it's fine the other way around), but that patch has already been committed.
OK?