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>

arch/xtensa/kernel/entry.S

@@ -2173,7 +2173,7 @@ ENDPROC(ret_from_kernel_thread)
 
 #ifdef CONFIG_HIBERNATION
 
-	.bss
+	.section	.bss, "aw"
 	.align	4
 .Lsaved_regs:
 #if defined(__XTENSA_WINDOWED_ABI__)