amdgcn: disallow USM on gfx908

It does work, but not well and only with the amdgpu.noreply=0 kernel boot
option.

gcc/ChangeLog:

	* config/gcn/gcn.cc (gcn_init_cumulative_args): Disallow gfx908.

diff --git a/gcc/config/gcn/gcn.cc b/gcc/config/gcn/gcn.cc
index 9c2fd4c5b8a..720c0a08a13 100644
--- a/gcc/config/gcn/gcn.cc
+++ b/gcc/config/gcn/gcn.cc
@@ -2905,6 +2905,7 @@ gcn_init_cumulative_args (CUMULATIVE_ARGS *cum /* Argument info to init */ ,
 	case PROCESSOR_FIJI:
 	case PROCESSOR_VEGA10:
 	case PROCESSOR_VEGA20:
+	case PROCESSOR_GFX908:
 	  error ("GPU architecture does not support Unified Shared Memory");
 	default:
 	  ;