target/sparc: Restore ldstub of odd asis

Fixes the booting of ss20 roms.

Cc: qemu-stable@nongnu.org
Reported-by: Michael Russo <mike@papersolve.com>
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>