Auto merge of #76415 - Mark-Simulacrum:bootstrap-cross-compilation, r=alexcrichton

rustbuild: avoid trying to inversely cross-compile for build triple from host triples

This changes rustbuild's cross compilation logic to better match what users expect,
particularly, avoiding trying to inverse cross-compile for the build triple from host triples.
That is, if build=A, host=B, target=B, we do not want to try and compile for A from B.
Indeed, the only "known to run" triple when cross-compiling is the build triple A.
When testing for a particular target we need to be able to run binaries compiled for
that target though.

The last commit also modifies the default set of host/target triples to avoid producing
needless artifacts for the build triple:

The new behavior is to respect --host and --target when passed as the *only*
configured triples (no triples are implicitly added). The default for --host is
the build triple, and the default for --target is the host triple(s), either
configured or the default build triple.

Fixes #76333

r? `@alexcrichton` if possible, otherwise we'll need to hunt down a reviewer

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