Remove std_detect dev dependency in core_arch

[Kobzol]

Instead of core_arch loading the is_XYZ_feature_enabled macros from the local std_detect crate, they are loaded from ::std::arch instead. This means that nothing in this repository will depend on std_detect anymore (except for tests, these will be ported later). The downside is that core_arch will need to wait for nightly std bumps to see changes in the detection macros, but I assume that should be rare.

This is one of the steps required to move std_detect fully into rust-lang/rust.

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[folkertdev]

changes to the detection macros should just happen in r-l/r then, right? At least when adding new target features that nicer anyway (in that case today we must wait for a nightly bump). It's only internal logic changes (which, as you say, are rare) that would get bitten by this, but then it should be easy enough to move such a PR over to r-l/r

[Kobzol]

changes to the detection macros should just happen in r-l/r then, right?

Yes. Hopefully the whole of std_detect will more into r-l/r soon-ish (rust-lang/rust#143412).