cpu: Register the ThreadContext in the CheckerCPU's ISA

This is needed after [1], as the Checker's ISA has an invalid (nullptr)
TC pointer.

[1]: https://gem5-review.googlesource.com/c/public/gem5/+/29233

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: I1d25096caf61f98451fca2d393d2ea1c4fca00e8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/46623
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/checker/cpu.cc

@@ -56,6 +56,7 @@ void
 CheckerCPU::init()
 {
     requestorId = systemPtr->getRequestorId(this);
+    tc->getIsaPtr()->setThreadContext(tc);
 }
 
 CheckerCPU::CheckerCPU(const Params &p)