fastmodel: Return nullptr from getCheckerCpuPtr on fast model CPUs.

Fast model CPUs won't (at least as of now) have a checker CPU attached
to them. We can safely return nullptr to signal that to calling code.

Change-Id: I7edd4f895d9c3767cb991a2b2af6538cf9661969
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24966
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/arch/arm/fastmodel/iris/thread_context.hh

@@ -195,11 +195,7 @@ class ThreadContext : public ::ThreadContext
     {
         return _dtb;
     }
-    CheckerCPU *
-    getCheckerCpuPtr() override
-    {
-        panic("%s not implemented.", __FUNCTION__);
-    }
+    CheckerCPU *getCheckerCpuPtr() override { return nullptr; }
     ArmISA::Decoder *
     getDecoderPtr() override
     {