CMake: Introduce Presets, replacing Settings

This introduces a CMakePresets file for both Linux and Windows that replace the old CMakeSettings.
It adds presets for **Debug** and **Release** profiles for both **x64** and **ARM64** architectures, as well as Generic builds, and can be used using Visual Studio's built-in CMakePresets support, or Visual Studio Code's CMake Tools extension.

They can also be used from the command line, like so:

x64/Linux:
Configure: `cmake --preset unix-release-x64`
Build: `cmake --build --preset unix-build-release-x64`
Configure + Build: `cmake --workflow --preset unix-release-x64`
ARM64/Windows:
Configure: `cmake --preset windows-release-arm64`
Build: `cmake --build --preset windows-build-release-arm64`
Configure + Build: `cmake --workflow --preset windows-release-arm64`

**Cross-compiling**

On Windows, the Visual Studio generator automatically takes care of selecting the cross-compiler if required.

On Linux, the Ninja generator is used. To cross-compile you need to install a cross-compiler and (optionally) a sysroot of the target system.
Here is an example to compile from x64 to ARM64 with a sysroot:

- `cmake --preset unix-release-arm64 -DCMAKE_C_COMPILER=aarch64-linux-gnu-gcc -DCMAKE_CXX_COMPILER=aarch64-linux-gnu-g++ -DCMAKE_SYSROOT=/opt/sysroots/aarch64-linux-gnu`
- `cmake --build --preset unix-build-release-arm64`

You will need a sysroot to link against Qt, since we do not vendor it in on platforms other than Windows.

A `CMakeUserPresets.json` file may be created locally at the root of the project to further customize your presets.
For example, here are the user presets I used to test this PR on Arch Linux with a generic Arch Linux ARM sysroot:

```json
{
  "version": 10,
  "configurePresets": [
    {
      "name": "gcc-debug-arm64",
      "inherits": "unix-debug-arm64",
      "cacheVariables": {
        "CMAKE_C_COMPILER": "aarch64-linux-gnu-gcc",
        "CMAKE_CXX_COMPILER": "aarch64-linux-gnu-g++",
        "CMAKE_EXE_LINKER_FLAGS": "-L/opt/sysroots/ArchLinuxARM/lib",
        "CMAKE_SYSROOT": "/opt/sysroots/ArchLinuxARM"
      }
    },
    {
      "name": "clang-debug-arm64",
      "inherits": "unix-debug-arm64",
      "cacheVariables": {
        "CMAKE_C_COMPILER": "clang",
        "CMAKE_CXX_COMPILER": "clang++",
        "CMAKE_C_FLAGS": "-target aarch64-linux-gnu",
        "CMAKE_CXX_FLAGS": "-target aarch64-linux-gnu",
        "CMAKE_SYSROOT": "/opt/sysroots/ArchLinuxARM"
      }
    }
  ],
  "buildPresets": [
    {
      "name": "gcc-build-debug-arm64",
      "configurePreset": "gcc-debug-arm64"
    },
    {
      "name": "clang-build-debug-arm64",
      "configurePreset": "clang-debug-arm64"
    }
  ],
  "workflowPresets": [
    {
      "name": "gcc-debug-arm64",
      "steps": [
        { "type": "configure", "name": "gcc-debug-arm64" },
        { "type": "build", "name": "gcc-build-debug-arm64" }
      ]
    },
    {
      "name": "clang-debug-arm64",
      "steps": [
        { "type": "configure", "name": "clang-debug-arm64" },
        { "type": "build", "name": "clang-build-debug-arm64" }
      ]
    }
  ]
}
```

They are then used like so:
Configure + Build with GCC: `cmake --workflow --preset gcc-debug-arm64`
Configure + Build with Clang: `cmake --workflow --preset clang-debug-arm64`

These changes should also make it trivial to cross-compile from Linux to FreeBSD and vice-versa, however this is untested.

Author: JoshuaVandaele

.gitignore

@@ -43,3 +43,5 @@ CMakeLists.txt.user
 /.vscode/
 # Ignore flatpak-builder's cache dir
 .flatpak-builder
+# Ignore CMake user presets
+CMakeUserPresets.json

CMake/DolphinToolchain.cmake

@@ -0,0 +1,50 @@
+set(ARCH "generic" CACHE STRING "Target architecture")
+
+set(VALID_ARCHS x86_64 arm64 generic)
+if(NOT ARCH IN_LIST VALID_ARCHS)
+    message(FATAL_ERROR "Invalid ARCH='${ARCH}'. Valid: ${VALID_ARCHS}")
+endif()
+
+if(ARCH STREQUAL "generic")
+    set(ENABLE_GENERIC "ON")
+else()
+    set(CMAKE_SYSTEM_PROCESSOR "${ARCH}")
+endif()
+
+if (NOT DEFINED CMAKE_SYSTEM_NAME)
+    if(EXISTS "${CMAKE_SYSROOT}/usr/include/linux")
+        set(CMAKE_SYSTEM_NAME Linux)
+    elseif(EXISTS "${CMAKE_SYSROOT}/Windows/System32")
+        set(CMAKE_SYSTEM_NAME Windows)
+    elseif(EXISTS "${CMAKE_SYSROOT}/System/Library")
+        set(CMAKE_SYSTEM_NAME Darwin)
+    elseif(EXISTS "${CMAKE_SYSROOT}/usr/include/osreldate.h")
+        set(CMAKE_SYSTEM_NAME FreeBSD)
+    elseif(EXISTS "${CMAKE_SYSROOT}/usr/include/c++/v1/__locale_dir/locale_base_api/openbsd.h")
+        set(CMAKE_SYSTEM_NAME OpenBSD)
+    elseif(EXISTS "${CMAKE_SYSROOT}/usr/include/dev/dm/netbsd-dm.h")
+        set(CMAKE_SYSTEM_NAME NetBSD)
+    elseif(EXISTS "${CMAKE_SYSROOT}/boot/system/develop")
+        set(CMAKE_SYSTEM_NAME Haiku)
+    else()
+        message(WARNING "Cannot detect OS from sysroot '${CMAKE_SYSROOT}/'. Cross-compilation has been disabled.")
+    endif()
+endif()
+
+set(CMAKE_FIND_ROOT_PATH "${CMAKE_SYSROOT}")
+set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
+set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
+
+if(NOT QT_HOST_PATH)
+    if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
+        if (CMAKE_HOST_SYSTEM_PROCESSOR MATCHES "AMD64")
+            set(QT_HOST_PATH "${CMAKE_SOURCE_DIR}/Externals/Qt/Qt6.5.1/x64")
+        else()
+            set(QT_HOST_PATH "${CMAKE_SOURCE_DIR}/Externals/Qt/Qt6.5.1/ARM64")
+        endif()
+    else()
+        set(QT_HOST_PATH "/usr")
+    endif()
+endif()