target-i386: Move TCG initialization check to tcg_x86_init()

Instead of requiring cpu.c to check if TCG was already initialized,
simply let the function be called multiple times.

Suggested-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>