Cortex-M3: use the new inheritance/nesting scheme

Cortex-M3: use the new inheritance/nesting scheme

Use new target_to_cm3() and target_to_armv7m() inlines,
instead of a series of x->arch_info conversions.  Remove
arch_info, since nothing uses it.

Also fix an omission:  the Cortex-M3 commands didn't verify
that they were operating on that kind of target.  Add comment
about the ARMv7M version of that omission.

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