Rollup merge of #144848 - Zalathar:ui-fulldeps, r=clubby789

For "stage 1" ui-fulldeps, use the stage 1 compiler to query target info

Testing ui-fulldeps in "stage 1" actually uses the stage 0 compiler, so that test programs can link against stage 1 rustc crates.

Unfortunately, using the stage 0 compiler causes problems when compiletest tries to obtain target information from the compiler, but the output format has changed since the last bootstrap beta bump.

We can work around this by also providing compiletest with a stage 1 compiler, and having it use that compiler to query for target information.

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This fixes the stage 1 ui-fulldeps failure seen at rust-lang/rust#144443 (comment).

Author: Zalathar