[cmake] more modules, general "cleanup" (#3126)

Successor to that old MoltenVK PR. Does a lot of cleanups within root CMakeLists.txt, hands over MoltenVK and VulkanValidationLayers to CPMUtil, and separates out common operations into my modules.

Hopefully reduces the monstrosity that is root CMakeLists.txt. Please test:

- builds on all platforms
- VulkanValidationLayers

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3126
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/dynarmic/CMakeModules/TargetArchitectureSpecificSources.cmake

@@ -1,5 +1,8 @@
+# SPDX-FileCopyrightText: Copyright 2025 Eden Emulator Project
+# SPDX-License-Identifier: GPL-3.0-or-later
+
 function(target_architecture_specific_sources project arch)
-    if (NOT DYNARMIC_MULTIARCH_BUILD)
+    if (NOT MULTIARCH_BUILD)
         target_sources("${project}" PRIVATE ${ARGN})
         return()
     endif()