Move target cpu tcg initialization to common code,
called from cpu_exec_realizefn.

Acked-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Emilio G. Cota <cota@braap.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>