Suspension fixes (#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.

src/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();
     }
 }
 

src/vm/threadsuspend.cpp

@@ -57,7 +57,7 @@ extern "C" void             RedirectedHandledJITCaseForGCStress_Stub(void);
 // Every PING_JIT_TIMEOUT ms, check to see if a thread in JITted code has wandered
 // into some fully interruptible code (or should have a different hijack to improve
 // our chances of snagging it at a safe spot).
-#define PING_JIT_TIMEOUT        10
+#define PING_JIT_TIMEOUT        1
 
 // When we find a thread in a spot that's not safe to abort -- how long to wait before
 // we try again.