target-arm: A64: Avoid duplicate exit_tb(0) in non-linked goto_tb

If gen_goto_tb() decides not to link the two TBs, then the
fallback path generates unnecessary code:
 * if singlestep is enabled then we generate unreachable code
   after the gen_exception_internal(EXCP_DEBUG)
 * if singlestep is disabled then we will generate exit_tb(0)
   twice, once in gen_goto_tb() and once coming out of the
   main loop with is_jmp set to DISAS_JUMP

Correct these deficiencies by only emitting exit_tb() in the
non-singlestep case, in which case we can use DISAS_TB_JUMP
to suppress the main-loop exit_tb().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
