target: create/use register_cache_invalidate()

target: create/use register_cache_invalidate()

Create a generic register_cache_invalidate(), and use it to
replace three all-but-identical core-specific routines:

 - armv4_5_invalidate_core_regs()
 - armv7m_invalidate_core_regs
 - mips32_invalidate_core_regs() too.

Make cache->num_regs be unsigned, avoiding various errors.

Net code shrink and simplification.

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