ARMv7-M: start using "struct arm"

ARMv7-M: start using "struct arm"

This sets up a few of the core "struct arm" data structures so they
can be used with ARMv7-M cores.  Specifically, it:

 - defines new ARM core_modes to match the microcontroller modes
   (e.g. HANDLER not IRQ, and two types of thread mode);

 - Establishes a new microcontroller "core_type", which can be
   used to make sure v7-M (and v6-M) cores are handled right;

 - adds "struct arm" to "struct armv7m" and arranges for the
   target_to_armv7m() converter to use it;

 - sets up the arm.core_cache and arm.cpsr values

 - makes the Cortex-M3 code maintain arm.map and arm.core_mode.

This is currently set up as a parallel data structure, primarily to
minimize special cases for the semihosting support with microcontroller
profile cores.

Later patches can rip out the duplicative ARMv7-M support and start
reusing core ARM code.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>