`download-rustc` should support a cross-compiled `host`

[jyn514]

Here is how download-rustc currently behaves when cross-compiling:

; x b --host i686-unknown-linux-gnu std
Building {std} stage2 library artifacts (x86_64-unknown-linux-gnu -> i686-unknown-linux-gnu)
Build completed successfully in 0:00:00
; x b --host i686-unknown-linux-gnu rustc
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `x86_64-unknown-linux-gnu`,
 right: `i686-unknown-linux-gnu`: Cross-compiling is not yet supported with `download-rustc`', compile.rs:1280:13
Build completed unsuccessfully in 0:00:00

Ideally, this would "just work": download rustc for stage 1, then rebuild stage 2 for the other target. This will be annoyingly slow because it also has to build llvm from source, but it's better than building both llvm and rustc twice.

[jyn514]

Found while investigating #110051 (comment).