xtensa: fix __sigsetjmp call12 case

Register window saving loop in __sigsetjmp incorrectly calculates offset
in the jmp_buf in case it was called with call12. The bug was introduced
in commit f4b1b7ade7971141 "xtensa: fix setjmp that didn't save all
registers correctly".

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

libc/sysdeps/linux/xtensa/setjmp.S

@@ -122,7 +122,7 @@ ENTRY (__sigsetjmp)
 	l32i	a4, a6, 12
 	s32i	a7, a5, 8
 	s32i	a4, a5, 12
-	addi	a5, a6, 16
+	addi	a5, a5, 16
 	addi	a6, a6, 16
 	blt	a6, a8, .Lsjloop
 .Lendsj: