Move cfg!(target_feature) directly into is_*_feature_detected!()

[bjorn3]

stdarch/crates/std_detect/src/detect/macros.rs

Lines 88 to 101 in d385078

	pub mod __is_feature_detected {
	$(
	/// PLEASE: do not use this, it is an implementation detail
	/// subject to change.
	#[inline]
	#[doc(hidden)]
	#[$stability_attr]
	pub fn $feature() -> bool {
	cfg!(target_feature = $feature_lit) ||
	$crate::detect::check_for($crate::detect::Feature::$feature)
	}
	)*
	}

It is currently part of a function inside the __is_feature_detected module of std_detect, which means that it uses the value of the target feature when compiling libstd instead of the target crate, making -Ctarget-cpu/-Ctarget-feature in RUSTFLAGS ineffective at optimizing the runtime detection away.