cpu_patches: Patch just-in-time using signal handlers. (#852)

* cpu_patches: Patch just-in-time using illegal instruction handler.

* core: Add common signal dispatch system and use for on-demand TCB patches.

* page_manager: Re-add userfaultfd implementation.

src/core/signals.h

@@ -0,0 +1,56 @@
+// SPDX-FileCopyrightText: Copyright 2024 shadPS4 Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#pragma once
+
+#include <set>
+#include "common/singleton.h"
+
+namespace Core {
+
+using AccessViolationHandler = bool (*)(void* code_address, void* fault_address, bool is_write);
+using IllegalInstructionHandler = bool (*)(void* code_address);
+
+/// Receives OS signals and dispatches to the appropriate handlers.
+class SignalDispatch {
+public:
+    SignalDispatch();
+    ~SignalDispatch();
+
+    /// Registers a handler for memory access violation signals.
+    void RegisterAccessViolationHandler(const AccessViolationHandler& handler, u32 priority) {
+        access_violation_handlers.emplace(handler, priority);
+    }
+
+    /// Registers a handler for illegal instruction signals.
+    void RegisterIllegalInstructionHandler(const IllegalInstructionHandler& handler, u32 priority) {
+        illegal_instruction_handlers.emplace(handler, priority);
+    }
+
+    /// Dispatches an access violation signal, returning whether it was successfully handled.
+    bool DispatchAccessViolation(void* code_address, void* fault_address, bool is_write) const;
+
+    /// Dispatches an illegal instruction signal, returning whether it was successfully handled.
+    bool DispatchIllegalInstruction(void* code_address) const;
+
+private:
+    template <typename T>
+    struct HandlerEntry {
+        T handler;
+        u32 priority;
+
+        std::strong_ordering operator<=>(const HandlerEntry& right) const {
+            return priority <=> right.priority;
+        }
+    };
+    std::set<HandlerEntry<AccessViolationHandler>> access_violation_handlers;
+    std::set<HandlerEntry<IllegalInstructionHandler>> illegal_instruction_handlers;
+
+#ifdef _WIN32
+    void* handle{};
+#endif
+};
+
+using Signals = Common::Singleton<SignalDispatch>;
+
+} // namespace Core