arm64: entry.S: move enable_step_tsk into kernel_exit

enable_step_tsk may enable single-step, so needs to mask debug
exceptions to prevent us from single-stepping kernel_exit. This
should be the callers problem.

Earlier cleanup (2a283070) moved disable_step_tsk into kernel_entry.
enable_step_tsk has two callers, both immediately before kernel_exit 0.
Move the macro call into kernel_exit after local_mask_daif.

enable_step_tsk is now only called with debug exceptions masked.
This was the last user of disable_dbg, remove it.

Signed-off-by: James Morse <james.morse@arm.com>