Remove #[no_mangle] from CORE_PERIPHERALS to enable zero-cost peripherals

[jonas-schievink]

The #[no_mangle] on the CORE_PERIPHERALS currently prevents LLVM from optimizing it out when only stored to:

cortex-m/src/peripheral/mod.rs

Lines 152 to 153 in 1c9d36e

	#[no_mangle]
	static mut CORE_PERIPHERALS: bool = false;

This means that even if an app only uses Peripherals::steal(), it still has to pay 1 Byte of RAM for the global. Since 1 is greater than 0, this can be improved.

#[no_mangle] is used to prevent multiple cortex-m versions to be linked together, so this behavior must be retained somehow. While we already use links in Cargo.toml for this, this only works with newer versions (that are currently not even released), so another (zero-sized) global should be introduced to take over CORE_PERIPHERALS' job.