log: Change log_cpu_state[_mask]() argument to CPUState

Make them available outside NEED_CPU_H and add documentation.

Signed-off-by: Andreas Färber <afaerber@suse.de>
cpu: Move reset logging to CPUState

x86 was using additional CPU_DUMP_* flags, so make that configurable in
CPUClass::reset_dump_flags.

This adds reset logging for alpha, unicore32 and xtensa.

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