Support building Rust and Cargo on a WASIX host

[kilyanni]

Building the Rust toolchain on WASIX currently needs downstream changes across
bootstrap host detection, Cargo platform handling, and git/curl dependencies.
The current patch combines those independent compatibility gaps and broadens
some behavior to every WebAssembly host.

Track the WASIX host port by capability, with a focused reproducer and test for
each bootstrap or Cargo failure. Keep WASIX-specific dependency policy and
platform branches scoped to the WASIX host target.

Related downstream patch:
Rust and Cargo WASIX host support