xtensa: change '.bss' to '.section .bss'

For some reason (ancient assembler?) the following build error is
reported by the kisskb:

  kisskb/src/arch/xtensa/kernel/entry.S: Error: unknown pseudo-op: `.bss':
  => 2176

Change abbreviated '.bss' to the full '.section .bss, "aw"' to fix this
error.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>