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>
target-alpha: Implement CPU reset

The parent_reset class field was already prepared but unused so far.

No guarantees that this actually does The Right Thing, more fields
may need to be moved within CPUAlphaState or to AlphaCPU.

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