Add some more builtins to chibicc

https://justine.lol/cosmopolitan/documentation.html should now contain a
lot of functions that had been missing previously due to not having them

third_party/chibicc/test/spinlock_test.c

@@ -0,0 +1,26 @@
+#include "third_party/chibicc/test/test.h"
+
+#define SPINLOCK(lock)                                \
+  do {                                                \
+    for (;;) {                                        \
+      typeof(*(lock)) x;                              \
+      __atomic_load(lock, &x, __ATOMIC_RELAXED);      \
+      if (!x && !__sync_lock_test_and_set(lock, 1)) { \
+        break;                                        \
+      } else {                                        \
+        __builtin_ia32_pause();                       \
+      }                                               \
+    }                                                 \
+  } while (0)
+
+#define SPUNLOCK(lock) __sync_lock_release(lock)
+
+_Alignas(64) char lock;
+
+main() {
+  ASSERT(0, lock);
+  SPINLOCK(&lock);
+  ASSERT(1, lock);
+  SPUNLOCK(&lock);
+  ASSERT(0, lock);
+}