scons: Handle TARGET_GPU_ISA not being set.

If TARGET_GPU_ISA is not set, even if the GPU ISA namespace isn't used
by anything, the logic which figures out what to set it to will fail.
This checks for that condition and sets it to something invalid, but
doesn't crash. If that namespace is actually used, then the build will
still fail.

Change-Id: Iec44255cccbafa4aceaa68bdd8b6a835dc0637a0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56895
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -563,7 +563,10 @@ env.Command('config/the_isa.hh', [],
 def makeTheGPUISA(source, target, env):
     gpu_isa = env['TARGET_GPU_ISA']
 
-    namespace = gpu_isa[0].upper() + gpu_isa[1:].lower() + 'ISA'
+    if gpu_isa:
+        namespace = gpu_isa[0].upper() + gpu_isa[1:].lower() + 'ISA'
+    else:
+        namespace = 'None'
 
     code = code_formatter()
     code('''\