FEXCore: Moves XID check to the frontend

The XID signal handler is OS specific and needs to be delegated to being
handled in the frontend. This only happens when we create a thread with
pthread rather than clone/fork/vfork.

The first created pthread will reset the XID signal handler in glibc,
which we need to check the first time this occurs. A little bit disjoint
while the actual thread creation still happens in the backend but will
come together once the thread creation gets moved to the frontend.

The only game I am aware of that requires this to work is SOMA since its
one of its middleware libraries tries asking for root privileges.

Retested the game to make sure it still works as expected.

Source/Tools/CommonTools/DummyHandlers.h

@@ -25,8 +25,6 @@ class DummySyscallHandler: public FEXCore::HLE::SyscallHandler, public FEXCore::
 
 class DummySignalDelegator final : public FEXCore::SignalDelegator, public FEXCore::Allocator::FEXAllocOperators {
   public:
-  void CheckXIDHandler() override {}
-
   void SignalThread(FEXCore::Core::InternalThreadState *Thread, FEXCore::Core::SignalEvent Event) override {}
 
   FEXCore::Core::InternalThreadState *GetBackingTLSThread() {