Add ARM64 builds and instructions

[lexcyn]

It is possible to build ruffle in native ARM64 for Windows using currently available tools so the instructions could be updated to include this, and since ARM64 runners are now generally available on Github, auto builds could be enabled.

To build manually, ensure:

• clang is installed and included in your PATH
• install latest OpenJDK arm64 from Microsoft + ensure it's added to PATH
• use the x64 version of Rust
• use ARM64 native toolchain compiler in MSVCC
• to build the release, use: cargo build --release --package=ruffle_desktop --target=aarch64-pc-windows-msvc

[torokati44]

• use the x64 version of Rust

Is this a typo? 🤔

[lexcyn]

• use the x64 version of Rust

Is this a typo? 🤔

There's no native arm64 version so the x64 version works as it does allow native cross compiling as long as your deps are native

[torokati44]

See: #21150 (comment)

[lexcyn]

I know this is probably not a priority, but it would be nice to have automated nightly builds for arm64 (I end up building them manually with each change) 😊