target-sparc: Introduce get_asi

Replace gen_get_asi, and use it for both 32-bit and 64-bit.
For v8, do supervisor and immediate checks here.

Also, move save_state and TB ending into the respective
subroutines, out of disas_sparc_insn.

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