cpu: Move cpu_copy() into linux-user

It is only used there and is deemed very fragile if not incorrect in its
current memcpy() form. Moving it into linux-user will allow to move
parts into target_cpu.h headers and only copy what the ABI mandates.

Signed-off-by: Andreas Färber <afaerber@suse.de>
cpu: Drop cpu_model_str from CPU_COMMON

Since this is only read in cpu_copy() and linux-user has a global
cpu_model, drop the field from generic code.

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