Rollup merge of #137072 - Urgau:check-cfg-load-builtins-at-once, r=Noratrieb

Load all builtin targets at once instead of one by one in check-cfg

This PR adds a method on `rustc_target::Target` to load all the builtin targets at once, and then uses that method when constructing the `target_*` values in check-cfg instead of load loading each target one by one by their name, which requires a lookup and was more of a hack anyway.

This may give us some performance improvements as we won't need to do the lookup for the _currently_ 287 targets we have.

Author: matthiaskrgr