Support Cortex M0 and M4 via ARCH_FLAGS

Stop foolishly overriding ARCH_FLAGS, and provide a warning text
for people who didn't set it.
Merge remote-tracking branch 'karl/locm3'

Conflicts:
	cpu/arm/stm32-ocm3/Makefile.stm32-ocm3

To stay in sync, pick Karl's variant of cpu/arm/stm32-ocm3/Makefile.stm32-ocm3
where the platform provides ARCH_FLAGS. This is not ideal, because it
requires the developer to know a lot about possibly rather arcane flags.
A possible solution could be to provide known to be consistent sets of
flags somewhere else, with the developer picking from that "menu".