Add atomics to chibicc

This change also fixes #434 and makes the chibicc assembler better.

third_party/chibicc/test/atomic_test.c

@@ -0,0 +1,41 @@
+#include "third_party/chibicc/test/test.h"
+
+_Atomic(int) lock;
+
+main() {
+  int x;
+
+  ASSERT(0, __atomic_exchange_n(&lock, 1, __ATOMIC_SEQ_CST));
+  __atomic_load(&lock, &x, __ATOMIC_SEQ_CST);
+  ASSERT(1, x);
+  ASSERT(1, __atomic_exchange_n(&lock, 2, __ATOMIC_SEQ_CST));
+  ASSERT(2, __atomic_exchange_n(&lock, 3, __ATOMIC_SEQ_CST));
+  ASSERT(3, __atomic_load_n(&lock, __ATOMIC_SEQ_CST));
+  __atomic_store_n(&lock, 0, __ATOMIC_SEQ_CST);
+  ASSERT(0, __atomic_fetch_xor(&lock, 3, __ATOMIC_SEQ_CST));
+  ASSERT(3, __atomic_fetch_xor(&lock, 1, __ATOMIC_SEQ_CST));
+  ASSERT(2, __atomic_load_n(&lock, __ATOMIC_SEQ_CST));
+
+  // CAS success #1
+  x = 2;
+  ASSERT(1, __atomic_compare_exchange_n(&lock, &x, 3, 0, __ATOMIC_SEQ_CST,
+                                        __ATOMIC_SEQ_CST));
+  ASSERT(2, x);
+  ASSERT(3, lock);
+
+  // CAS success #2
+  x = 3;
+  ASSERT(1, __atomic_compare_exchange_n(&lock, &x, 4, 0, __ATOMIC_SEQ_CST,
+                                        __ATOMIC_SEQ_CST));
+  ASSERT(3, x);
+  ASSERT(4, lock);
+
+  // CAS fail
+  x = 3;
+  ASSERT(0, __atomic_compare_exchange_n(&lock, &x, 7, 0, __ATOMIC_SEQ_CST,
+                                        __ATOMIC_SEQ_CST));
+  ASSERT(4, x);
+  ASSERT(4, lock);
+
+  //
+}