arch-gcn3: Modify directory structure as prep for adding vega isa

Change-Id: I7c5f4a3a9d82ca4550e833dec2cd576dbe333627
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42203
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>

src/arch/SConscript

@@ -67,14 +67,17 @@ env.SwitchingHeaders(
         '''),
     env.subst('${TARGET_ISA}'))
 
-if env['BUILD_GPU']:
-    env.SwitchingHeaders(
-        Split('''
-            gpu_decoder.hh
-            gpu_isa.hh
-            gpu_types.hh
-            '''),
-        env.subst('${TARGET_GPU_ISA}'))
+amdgpu_isa = ['gcn3']
+
+env.SwitchingHeaders(
+    Split('''
+        gpu_decoder.hh
+        gpu_isa.hh
+        gpu_registers.hh
+        gpu_types.hh
+        '''),
+    '{}'.format('amdgpu/' if env['TARGET_GPU_ISA'] in amdgpu_isa else '')+
+    env.subst('${TARGET_GPU_ISA}'))
 
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