Auto merge of #107080 - Urgau:cleanup-bootstrap-extra-check-cfgs, r=Mark-Simulacrum

bootstrap: cleanup the list of extra check cfgs

This PR performs some cleanups on the `EXTRA_CHECK_CFGS` list in bootstrap.

- `target_os=watchos`: no longer relevant because there are now proper targets `*-apple-watchos`
 - `target_arch=nvptx64`: target `nvptx64-nvidia-cuda` makes it useless
 - `target_arch=le32`: target was removed (rust-lang/rust#45041)
 - `release`: was removed from rustfmt (rust-lang/rustfmt#5375 and rust-lang/rustfmt#5449)
 - `dont_compile_me`: was removed from stdarch (rust-lang/stdarch#1308)

Also made some external cfg exception mode clear and only activated for rustc and rustc tools (as to not have the Standard Library unintentionally depend on them).

Author: bors