[D3D12] Add support for building with MinGW.

[bruvzg]

Some godot-nir-static changes required, see godotengine/godot-nir-static#2 (for MinGW only, MSVC build should work without it).

Tested building and running on:

• x86_64 with MinGW/GCC (cross build from macOS and native MSYS2 build).
• x86_64 with MinGW/LLVM (cross build from macOS).

Compiling docs update (godotengine/godot-docs#8624)

Note: PIX do not have MinGW import libs, but it's possible to generate it using following commands (from PIX runtime folder):

gendef ./bin/x64/WinPixEventRuntime.dll
dlltool --machine i386:x86-64 --no-leading-underscore -d WinPixEventRuntime.def -D WinPixEventRuntime.dll -l ./bin/x64/libWinPixEventRuntime.a

[bruvzg]

Superseded by #83452