cpu: Introduce CPUClass::memory_rw_debug() for target_memory_rw_debug()

Make inline target_memory_rw_debug() always available and change its
argument to CPUState. Let it check if CPUClass::memory_rw_debug provides
a specialized callback and fall back to cpu_memory_rw_debug() otherwise.

The only overriding implementation is for 32-bit sparc.

This prepares for changing GDBState::g_cpu to CPUState.

Signed-off-by: Andreas Färber <afaerber@suse.de>
