Fix test failure

Somehow a test reached Thread::CooperativeCleanup() with m_pRuntimeThreadLocals==NULL. Looking at the code I expected that would mean GetThread()==NULL or ThreadState contains TS_Dead, but neither of those conditions were true so it is unclear how it executed to that state. The callstack was:
>	coreclr.dll!Thread::CooperativeCleanup() Line 2762	C++
 	coreclr.dll!Thread::DetachThread(int fDLLThreadDetach) Line 936	C++
 	coreclr.dll!TlsDestructionMonitor::~TlsDestructionMonitor() Line 1745	C++
 	coreclr.dll!__dyn_tls_dtor(void * __formal, const unsigned long dwReason, void * __formal) Line 122	C++
 	ntdll.dll!_LdrxCallInitRoutine@16()	Unknown
 	ntdll.dll!LdrpCallInitRoutine()	Unknown
 	ntdll.dll!LdrpCallTlsInitializers()	Unknown
 	ntdll.dll!LdrShutdownThread()	Unknown
 	ntdll.dll!RtlExitUserThread()	Unknown

Regardless, the previous code wouldn't have hit that issue because it obtained the pointer through t_runtime_thread_locals rather than m_pRuntimeThreadLocals. I restored using t_runtime_thread_locals in the Cleanup routine. Out of caution I also searched for any other places that were previously accessing the alloc_context through the thread local address and ensured they don't switch to use m_pRuntimeThreadLocals either.

Author: noahfalk

src/coreclr/vm/threads.cpp

@@ -2755,7 +2755,7 @@ void Thread::CooperativeCleanup()
 
     if (GCHeapUtilities::IsGCHeapInitialized())
     {
-        gc_alloc_context* gc_alloc_context = GetAllocContext(); 
+        gc_alloc_context* gc_alloc_context = &t_runtime_thread_locals.alloc_context.gc_allocation_context; 
         // If the GC heap is initialized, we need to fix the alloc context for this detaching thread.
         // GetTotalAllocatedBytes reads dead_threads_non_alloc_bytes, but will suspend EE, being in COOP mode we cannot race with that
         // however, there could be other threads terminating and doing the same Add.

src/coreclr/vm/threadsuspend.cpp

@@ -2378,7 +2378,7 @@ void Thread::PerformPreemptiveGC()
         // BUG(github #10318) - when not using allocation contexts, the alloc lock
         // must be acquired here. Until fixed, this assert prevents random heap corruption.
         _ASSERTE(GCHeapUtilities::UseThreadAllocationContexts());
-        GCHeapUtilities::GetGCHeap()->StressHeap(GetAllocContext());
+        GCHeapUtilities::GetGCHeap()->StressHeap(&t_runtime_thread_locals.alloc_context.gc_allocation_context);
         m_bGCStressing = FALSE;
     }
     m_GCOnTransitionsOK = TRUE;