target-ppc: Introduce unrealizefn for PowerPCCPU

Use it to clean up the opcode table, resolving a former TODO from Jocelyn.
Also switch from malloc() to g_malloc().

Signed-off-by: Andreas Färber <afaerber@suse.de>
target-ppc: Drop unnecessary dynamic cast in ppc_env_get_cpu()

A transition from CPUPPCState to PowerPCCPU can be considered safe,
just like PowerPCCPU::env access in the opposite direction.

This should slightly improve interrupt performance.

Reported-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>