Named local labels shouldn't be used in LLVM because they might get
duplicated resulting in multiple symbol definitions and a compiler
error. Instead, use numeric labels.
This commit adds support for the nrf52, which is a Cortex-M4 running at
64MHz.
While implementing this, I discovered that the timings for AVR/16MHz and
Cortex-M0 at 16MHz weren't quite right. They do work, but were slightly
out of spec. So I fixed this as well.
Ayke van Laethem