[committed, amdgcn] Disable trampolines on GCN5

[Andrew Stubbs]

On 22/05/2019 23:21, Andrew Stubbs wrote:
> Trampolines work just fine on GCN3 devices, but GCN5 devices have 
> execute permission hardware, and the driver sets permission off for the 
> private segment allocation in which the stacks are located.
> 
> It may be possible to implement indirect calls to nested functions 
> another way, but for now this will prevent unexplained crashes or hangs 
> on unsupported devices.

This is now backported to gcc-9-branch.

Andrew