target-sparc: Unify asi handling between 32 and 64-bit

We now have a single copy of gen_ld_asi, gen_st_asi,
gen_swap_asi, and everything uses gen_get_asi.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>