Add a <stdatomic.h> header test, and the missing kill_dependency macro.

Test expectations based on
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdatomic.h.html

Change-Id: Ida47c3a9c67a9c4af8a4c4174d2e765615034175
diff --git a/libc/include/bits/stdatomic.h b/libc/include/bits/stdatomic.h
index c74eafd..ebdc9e5 100644
--- a/libc/include/bits/stdatomic.h
+++ b/libc/include/bits/stdatomic.h
@@ -134,6 +134,8 @@
 	memory_order_seq_cst = __ATOMIC_SEQ_CST
 } memory_order;
 
+#define kill_dependency(y) (y)
+
 /*
  * 7.17.4 Fences.
  */