target/ppc: move opcode table logic to translate.c

code motion to remove opcode callback table from
translate_init.c.inc to translate.c in preparation to remove
the #include <translate_init.c.inc> from translate.c. Also created
destroy_ppc_opcodes and removed that logic from ppc_cpu_unrealize

Signed-off-by: Bruno Larsen (billionai) <bruno.larsen@eldorado.org.br>
Message-Id: <20210429162130.2412-2-bruno.larsen@eldorado.org.br>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>