Cortex-M4F Architecture

Architecture support for Cortex-M4F devices. This largely only re-exports the correct functions from the Cortex-M crate.

Note: Mainline Tock does not currently have any support for hard FPUs. This is currently a direct clone of the cortexm4. However, chips wil FPUs should point to this arch crate to pick up hard float support when it is integrated.