Windows: Use FrontendPtr for extra TLS space on WOW64/ARM64EC

We have run out of EmulatorData slots on ARM64EC, so introduce this for
less-frequently accessed TLS data for which the additional cost doesn't
matter.

Source/Windows/ARM64EC/Module.cpp

@@ -126,6 +126,10 @@ struct ThreadCPUArea {
   }
 };
 
+struct FrontendThreadData {
+  bool InLockedRWXRead {};
+};
+
 namespace {
 fextl::unique_ptr<FEXCore::Context::Context> CTX;
 fextl::unique_ptr<FEX::DummyHandlers::DummySignalDelegator> SignalDelegator;
@@ -150,6 +154,10 @@ ThreadCPUArea GetCPUArea() {
   return ThreadCPUArea(NtCurrentTeb());
 }
 
+FrontendThreadData* GetFrontendThreadData(FEXCore::Core::InternalThreadState* Thread) {
+  return static_cast<FrontendThreadData*>(Thread->FrontendPtr);
+}
+
 bool IsEmulatorStackAddress(const ThreadCPUArea CPUArea, uint64_t Address) {
   return Address <= CPUArea.EmulatorStackBase() && Address >= CPUArea.EmulatorStackLimit();
 }
@@ -928,6 +936,8 @@ NTSTATUS ThreadInit() {
                             .AMD64_ControlWord = 0x27f};
   Exception::LoadStateFromECContext(Thread, CPUArea.ContextAmd64().AMD64_Context);
 
+  Thread->FrontendPtr = new FrontendThreadData();
+
   {
     auto ThreadTID = GetCurrentThreadId();
     Threads.emplace(ThreadTID, Thread);
@@ -983,6 +993,8 @@ NTSTATUS ThreadTerm(HANDLE Thread, LONG ExitCode) {
   }
   auto ThreadState = CPUArea.ThreadState();
 
+  delete GetFrontendThreadData(ThreadState);
+
   // GDT and LDT are mirrored, only free one.
   delete[] ThreadState->CurrentFrame->State.segment_arrays[FEXCore::Core::CPUState::SEGMENT_ARRAY_INDEX_GDT];