ARMv7-M: make DAP commands verify target is an ARMv7-M

Init the ARMv7-M magic number.  Define predicate verifying it.
Use it to resolve a lurking bug/FIXME:  make sure the ARMv7-M
specific DAP ops reject non-ARMv7-M targets.

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