build should not bootstrap compilers for non-build-triple host compilers

[brson]

When building a cross-compiled Rust compiler, e.g. building a rustc that runs on i686 from x86_64, we first bootstrap the 'A' compiler (x86), from stage0 to stage2, then we use the stage2 'A' compiler to build the stage0 'B' compiler (i686), and go through the entire bootstrap process again, building the 'B' compiler with itself.

This has some problems:

1. It's inefficient, bootstrapping two compilers
2. It's not necessary since the first 'B' compiler is built from a compiler that is already up to date
3. It only works when 'A' is x86_64 and 'B' is i686 (or vice-versa), and both for the same OS, since there's no reason to assume that the 'B' compiler can even run on the 'A' compiler's architecture.

Fixing this is going to require some deep restructuring of the makefiles.

[bstrie]

Since the build system is eventually moving from Make to Rust, this should not be an immediate priority.

[brson]

Don't know why this is on a milestone. Removing.

[lucab]

As a sidenote, this could be interesting for Debian in order to make the whole system cross-bootstrappable. Not in the short term, though.

[huonw]

part of #8058

[steveklabnik]

Triage:

Fixing this is going to require some deep restructuring of the makefiles

I would guess this is why there's no change so far.

[bstrie]

Triage: still relevant, though rustbuild may be upon us at last to free us from our woes: #31590

[alexcrichton]

This is now done with rustbuild, (the future!), so closing.