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>

src/target/cortex_m3.h

@@ -161,7 +161,6 @@ typedef struct cortex_m3_common_s
 	struct reg_cache_s *dwt_cache;
 
 	armv7m_common_t armv7m;
-	void *arch_info;
 } cortex_m3_common_t;
 
 static inline struct cortex_m3_common_s *