Incorporate DWT Tracing helpers from stm32f4xx-hal?

[astapleton]

The stm32f4xx-hal has a neat module for using the DWT for tracing/profiling (https://github.com/stm32-rs/stm32f4xx-hal/blob/master/src/dwt.rs). Given that the DWT is a Cortex-M feature, I was wondering if it makes sense to move it here and expose it either via the cortex-m crate, or a new crate? It could a neat thing to build into a helpful tool for all Cortex-M developers.