[committed, amdgcn] Wait for exit value to write before exiting.

[Andrew Stubbs]

On 24/05/2019 16:31, Andrew Stubbs wrote:
> This patch fixes a bug in which GCN5 devices often fail to return an 
> exit value because it's not yet been written to memory when the program 
> exits. The fix is simply to wait for it properly. GCN3 devices did not 
> demonstrate the problem, but it was technically wrong there also. The 
> bug was introduced when we stopped waiting for all writes to complete.
> 
> I've also taken the opportunity to adjust gcn-run such that a similar 
> issue can't go unnoticed for so long, in future.

Now backported to gcc-9-branch.

Andrew