target-arm: Split A64 from A32/T32 gen_intermediate_code_internal()

The A32/T32 gen_intermediate_code_internal() is complicated because it
has to deal with:
 * conditionally executed instructions
 * Thumb IT blocks
 * kernel helper page
 * M profile exception-exit special casing

None of these apply to A64, so putting the "this is A64 so
call the A64 decoder" check in the middle of the A32/T32
loop is confusing and means the A64 decoder's handling of
things like conditional jump and singlestepping has to take
account of the conditional-execution jumps the main loop
might emit.

Refactor the code to give A64 its own gen_intermediate_code_internal
function instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>