cpu: Make getIsaPtr const.

This accessor shouldn't modify the ThreadContext.

Change-Id: I6f2629f950b0379573e51e337626ef6b1429a14d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50250
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/thread_context.hh

@@ -142,7 +142,7 @@ class ThreadContext : public PCEventScope
 
     virtual CheckerCPU *getCheckerCpuPtr() = 0;
 
-    virtual BaseISA *getIsaPtr() = 0;
+    virtual BaseISA *getIsaPtr() const = 0;
 
     virtual InstDecoder *getDecoderPtr() = 0;