cpu: Avoid QOM casts for CPU()

CPU address spaces touching load and store helpers as well as the
movement of (almost) all fields from CPU_COMMON to CPUState have led to
a noticeable increase of CPU() usage in "hot" paths for both TCG and KVM.

While CPU()'s OBJECT_CHECK() might help detect development errors, i.e.
in form of crashes due to QOM vs. non-QOM mismatches rather than QOM
type mismatches, it is not really needed at runtime since mostly used in
CPU-specific paths, coming from a target-specific CPU subtype. If that
pointer is damaged, other errors are highly likely to occur elsewhere
anyway.

Keep the CPU() macro for a consistent developer experience and for
flexibility to exchange its implementation, but turn it into a pure,
unchecked C cast for now.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>