Cortex-M3: don't chain "struct arm" commands

Cortex-M3: don't chain "struct arm" commands

Those commands presume support for the "classic" set of CPU
modes (FIQ, supervisor, IRQ, etc) ... which aren't supported
by the ARMv7-M or ARMv6-M architectures.  They also presume
a "struct arm" base type, which this code doesn't use.

We haven't cleaned up the register handling enough to be able
to share any of those "base" methods.

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