Cleanup free functions

[wt]

My asm refactor allows us to get rid of

cortex-m/cortex-m/src/interrupt.rs

Lines 90 to 102 in 6baaf6a

	// Make a `free()` function available on hosted platforms to allow checking dependencies without
	// specifying a target, but that will panic at runtime if executed.
	/// Execute closure `f` in an interrupt-free context.
	///
	/// This as also known as a "critical section".
	#[cfg(not(cortex_m))]
	#[inline]
	pub fn free<F, R>(_: F) -> R
	where
	F: FnOnce(&CriticalSection) -> R,
	{
	panic!("cortex_m::interrupt::free() is only functional on cortex-m platforms");
	}

and remove the cfg guard from the normal free function.

The normal free function is already compileable on non-cortex-m because it relies on functions that will simply call unimplemented!() if they are built on unsupported targets.