oldland: fix register parsing for prefix names.

$r1 is a prefix of $r10, but as we were only comparing up to the end of
$r1 we would incorrectly match and then the assembler would barf as the
next token would start with the 0.  Sort the registers by length so we
process $r10-$r12 before $r1.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>