Intent to ship: Large ArrayBuffers and large wasm memories

[Lars Hansen]

In Firefox 89 I intend to turn on support for large ArrayBuffers/SharedArrayBuffers (up to 8GB, but easily adjustable) and large WebAssembly memories (up to 4GB) for the x64 and arm64 platforms. It has been developed behind the `javascript.options.large_arraybuffers` preference. Chromium/V8 has supported large ArrayBuffers and memories for some time already.

Previously, our limit on ArrayBuffers and wasm memories was 2GB, but this limit is a hardship for  WebAssembly applications that want to use more memory than that for asset caching in particular.

DOM APIs are still limited to 2GB views though those views can be onto larger underlying buffers; if raw ArrayBuffers are passed to DOM, they are limited to 2GB.  We will fix this if/when it turns out to be a hardship.

On 32-bit platforms, on mips64, and for optimized asm.js compilation the ArrayBuffer limit remains 2GB.

Bug to turn on by default: https://bugzilla.mozilla.org/show_bug.cgi?id=1703505

--lars