Suspension fixes (dotnet/coreclr#27578)

* Two simple fixes to suspension issues seen in GCPerfSim:

- Insert allow_fgc() call in background_mark_simple - this fixes the cases where there are a ton of GC handles referencing simple objects not containing pointers.

- Change PING_JIT_TIMEOUT constant from 10 milliseconds to 1 millisecond. This fixes the case where return address hijacking doesn't work quickly, because the hijacked thread doesn't return (e.g. because it's in a loop doing further calls). In this case we have to retry the hijack, and changing the timeout constant makes this happen more quickly.

Commit migrated from dotnet/coreclr@fab7aa2

src/Native/gc/gc.cpp

@@ -19982,6 +19982,7 @@ gc_heap::background_mark_simple (uint8_t* o THREAD_NUMBER_DCL)
                 background_mark_simple1 (o THREAD_NUMBER_ARG);
             }
         }
+        allow_fgc();
     }
 }