Fix macOS AU build setup

[lpla]

Make the DPF branch build a macOS AU plugin reliably on current macOS.

• Prefer Apple framework locations for SDL2.framework before falling back to pkg-config SDL2.
• Use portable CPU-count detection in build.sh on macOS.
• Allow RPC schema generation and tjsc bytecode generation to use host-built tools when building Apple target architectures that do not include the host architecture.
• Build the SameBoy pb12 helper for the host architecture on Apple so arm64-only target builds can still run the helper on Intel hosts.
• Update submodules for the DPF, rpcpp, and lv_binding_js fixes.

Depends on:

• Fix macOS AU build setup DPF#1
• Avoid macOS stdio header collision rpcpp#2
• Fix macOS AU build issues lv_binding_js#2
• Fix macOS plugin build issues saghul/txiki.js#955

Validation:

• Built retroplug-au for x86_64 and validated with auval -v aumu RPlg Dstr.
• Built retroplug-au for arm64 using host-built rpc-schema-dump, tjsc, and retroplug-export overrides.
• Built retroplug-au as universal x86_64;arm64; lipo confirmed both slices in the AU binary and helper tools.
• Codesigned and installed the universal AU to ~/Library/Audio/Plug-Ins/Components/retroplug.component; auval -v aumu RPlg Dstr succeeded on the Intel host.

[tommitytom]

Thanks for this! I'll give this a proper look over the next couple of days - I'm a bit weary of the changes to DPF since that should already support MacOS and ideally I'd like to not be using a fork of it. Also worth mentioning the dpf branch is a work in progress, there are a lot of missing features. Don't trust your projects to it as you may experience data loss!