target/arm: [a64] Move page and ss checks to init_disas_context

Since AArch64 uses a fixed-width ISA, we can pre-compute the number of
insns remaining on the page.  Also, we can check for single-step once.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>