sim: Move findFreeContext to System.

This method searches through the ThreadContexts stored in the system,
and has no concrete connection to Process other than it happened to be
used by a Process in the clone system call.

By moving it, we can use its functionality in classes other than
Process.

Change-Id: Ic6899c335dc13841c6fe948ea3a4f8ad67e562bb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24285
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/system.hh

@@ -188,6 +188,14 @@ class System : public SimObject, public PCEventScope
     unsigned int cacheLineSize() const { return _cacheLineSize; }
 
     std::vector<ThreadContext *> threadContexts;
+    ThreadContext *findFreeContext();
+
+    ThreadContext *
+    getThreadContext(ContextID tid) const
+    {
+        return threadContexts[tid];
+    }
+
     const bool multiThread;
 
     using SimObject::schedule;
@@ -195,11 +203,6 @@ class System : public SimObject, public PCEventScope
     bool schedule(PCEvent *event) override;
     bool remove(PCEvent *event) override;
 
-    ThreadContext *getThreadContext(ContextID tid) const
-    {
-        return threadContexts[tid];
-    }
-
     unsigned numContexts() const { return threadContexts.size(); }
 
     /** Return number of running (non-halted) thread contexts in