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>
cpu: Unify reset handling

As an interim step towards cleaning up CPU reset, reset in CPUState's
realizefn. This adds reset calls for alpha and unicore32, who do not
implement memset(env, 0, offsetof(CPUArchState, breakpoints)) yet.
Also now reset are ppc and sparc.

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