Make threading lock-free

The core and the headless tools no longer use mutexes, condition variables,
spinlocks, std::atomic, std::thread or volatile flags. Threads hand off through
libretro-common's retro_atomic and rthreads plus a few small pieces on top of
them in Common/Thread: Thread, MpscQueue, EventCounter, ParkingLot waits and
ReaderGate. Every wait blocks until it's woken; nothing polls on a timer.

State that only one thread touches just loses its lock. The PSP filesystem,
sceIo and the savedata dialog now belong to the emulation thread: async reads
and writes run when they're queued and complete later in emulated time, and
the IO and savedata threads are gone. The friend finder only talks to the
adhoc server and hands what it gets to the emulation thread. GPU init always
runs on the thread that starts the game.

The Vulkan render and compile queues, pipeline and render pass caches, delete
list and SPIR-V cache, the GL render queue, the software renderer's jit caches,
the file loader caches, the web debugger, UPnP, input, savestates, logging and
the GE debugger all hand off through queues and atomics. The IR jit's spinning
profiler thread goes.

A CPU thread with nothing to run now sleeps until work is queued instead of
spinning. Headless with --debugger waits for a client while stopped, and ends
the run once every client that connected has left. Web server shutdown keeps
servicing the CPU queue until the server has finished.

Core/HLE/sceKernelThread.cpp

@@ -18,7 +18,6 @@
 #include <algorithm>
 #include <list>
 #include <map>
-#include <mutex>
 #include <set>
 
 #include "Common/CommonTypes.h"
@@ -531,8 +530,6 @@ static std::vector<SceUID> pendingDeleteThreads;
 
 // Lists all thread ids that aren't deleted/etc.
 static std::vector<SceUID> threadqueue;
-// Only for debugger, so not needed to read, just write.
-static std::mutex threadqueueLock;
 
 // Lists only ready thread ids.
 static ThreadQueueList threadReadyQueue;
@@ -1098,8 +1095,6 @@ void __KernelIdle()
 }
 
 void __KernelThreadingShutdown() {
-	std::lock_guard<std::mutex> guard(threadqueueLock);
-
 	kernelMemory.Free(threadReturnHackAddr);
 	threadqueue.clear();
 	threadReadyQueue.clear();
@@ -1608,8 +1603,6 @@ void __KernelCancelThreadEndTimeout(SceUID threadID)
 }
 
 static void __KernelRemoveFromThreadQueue(SceUID threadID) {
-	std::lock_guard<std::mutex> guard(threadqueueLock);
-
 	int prio = __KernelGetThreadPrio(threadID);
 	if (prio != 0)
 		threadReadyQueue.remove(prio, threadID);
@@ -1993,8 +1986,6 @@ void __KernelResetThread(PSPThread *t, int lowestPriority) {
 }
 
 PSPThread *__KernelCreateThreadObject(SceUID &id, SceUID moduleId, const char *name, u32 entryPoint, u32 priority, int stacksize, u32 attr, BlockAllocator *stackAllocator) {
-	std::lock_guard<std::mutex> guard(threadqueueLock);
-
 	PSPThread *t = new PSPThread();
 	id = kernelObjects.Create(t);
 
@@ -3886,7 +3877,6 @@ void __KernelRegisterWaitTypeFuncs(WaitType type, WaitBeginCallbackFunc beginFun
 }
 
 std::vector<DebugThreadInfo> GetThreadsInfo() {
-	std::lock_guard<std::mutex> guard(threadqueueLock);
 	std::vector<DebugThreadInfo> threadList;
 
 	u32 error;