xtensa: fix stack dump output

Use %pB in pr_cont format string instead of calling print_symbol
separately. It turns

  [   19.166249] Call Trace:
  [   19.167265]  [<a000e50a>]
  [   19.167843]  __warn+0x92/0xa0
  [   19.169656]  [<a000e554>]
  [   19.170059]  warn_slowpath_fmt+0x3c/0x40
  [   19.171934]  [<a02d5bd8>]

back into

  [   18.123240] Call Trace:
  [   18.125039]  [<a000e4f6>] __warn+0x92/0xa0
  [   18.126961]  [<a000e540>] warn_slowpath_fmt+0x3c/0x40

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