base: singleStep can't be interupted by trap from other thread.

Change-Id: I0c46e3ea623b304b7ae8f8867d90c5d0008e8b3f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63533
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/remote_gdb.cc

@@ -555,6 +555,12 @@ BaseRemoteGDB::trap(ContextID id, int signum,const std::string& stopReason)
         return;
 
     if (tc->contextId() != id) {
+
+        //prevent thread switch when single stepping
+        if (singleStepEvent.scheduled()){
+            return;
+        }
+        DPRINTF(GDBMisc, "Finishing thread switch");
         if (!selectThreadContext(id))
             return;
     }