Custom Target Tracking

[daxpedda]

This is tracking work for our own Rust target.

• Wire up JS file output in Cargo.
• Implement asm! interpolate rust-lang/rust#156719
• Inline ASM support to generate object files instead of passing to LLVM.
• Custom target JSON support for CI/local development without a custom toolchain. Custom source standard library rust-lang/wg-cargo-std-aware#7