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 (https://github.com/rust-lang/rust/pull/45041)
- `release`: was removed from rustfmt (https://github.com/rust-lang/rustfmt/pull/5375 and https://github.com/rust-lang/rustfmt/pull/5449)
- `dont_compile_me`: was removed from stdarch (https://github.com/rust-lang/stdarch/pull/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).
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