[XTENSA] Remove non-rt signal handling

The non-rt signal handling was never really used, so we don't break
anything. This patch also cleans up the signal stack-frame to make
it independent from the processor configuration. It also improves
the method used for controlling single-stepping. We now save and
restore the 'icountlevel' register that controls single stepping
and set or clear the saved state to enable or disable it.

Signed-off-by: Chris Zankel <chris@zankel.net>
diff --git a/include/asm-xtensa/unistd.h b/include/asm-xtensa/unistd.h
index 8a7fb69..9bd3402 100644
--- a/include/asm-xtensa/unistd.h
+++ b/include/asm-xtensa/unistd.h
@@ -485,8 +485,8 @@
 __SYSCALL(218, sys_sched_rr_get_interval, 2)
 #define __NR_sched_yield 			219
 __SYSCALL(219, sys_sched_yield, 0)
-#define __NR_sigreturn	 			222
-__SYSCALL(222, xtensa_sigreturn, 0)
+#define __NR_available222 			222
+__SYSCALL(222, sys_ni_syscall, 0)
 
 /* Signal Handling */