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>