xtensa: improve stack dumping

Calculate printable stack size and use print_hex_dump instead of
opencoding it.
Drop extra newline output in show_trace as its output format does not
depend on CONFIG_KALLSYMS.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>