cpu: add CPU_RESOLVING_TYPE macro

cpu: add CPU_RESOLVING_TYPE macro

it will be used for providing to cpu name resolving class for
parsing cpu model for system and user emulation code.

Along with change add target to null-machine tests, so
that when switch to CPU_RESOLVING_TYPE happens,
it would ensure that null-machine usecase still works.

Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]> (m68k)
Acked-by: David Gibson <[email protected]> (ppc)
Acked-by: Bastian Koppelmann <[email protected]> (tricore)
Message-Id: <1518000027[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>
[ehabkost: Added macro to riscv too]
Signed-off-by: Eduardo Habkost <[email protected]>