xtensa: rearrange show_stack output

Minimal stack alignment on xtensa is 16 bytes, having stack dump in
32-byte lines may be visually misleading as the stack frame border may
be in the middle of the line.
Arrange stack dump in 16-byte lines. Mark lines at stack frame borders
with arrows.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>