target-unicore32: Move TCG initialization to UniCore32CPU initfn

Normalize the "inited" logic and add a tcg_enabled() check to suppress
it for qtest.

Ensures that a QOM-created UniCore32CPU is usable.

Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Andreas Färber <afaerber@suse.de>
