Add chibicc

This program popped up on Hacker News recently. It's the only modern
compiler I've ever seen that doesn't have dependencies and is easily
modified. So I added all of the missing GNU extensions I like to use
which means it might be possible soon to build on non-Linux and have
third party not vendor gcc binaries.

third_party/chibicc/test/varargs_test.c

@@ -0,0 +1,52 @@
+#include "third_party/chibicc/test/test.h"
+
+int sum1(int x, ...) {
+  va_list ap;
+  va_start(ap, x);
+
+  for (;;) {
+    int y = va_arg(ap, int);
+    if (y == 0) return x;
+    x += y;
+  }
+}
+
+int sum2(int x, ...) {
+  va_list ap;
+  va_start(ap, x);
+
+  for (;;) {
+    double y = va_arg(ap, double);
+    x += y;
+
+    int z = va_arg(ap, int);
+    if (z == 0) return x;
+    x += z;
+  }
+}
+
+void fmt(char *buf, char *fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+
+  va_list ap2;
+  va_copy(ap2, ap);
+  vsprintf(buf, fmt, ap2);
+  va_end(buf);
+}
+
+int main() {
+  ASSERT(6, sum1(1, 2, 3, 0));
+  ASSERT(55, sum1(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0));
+  ASSERT(21, sum2(1, 2.0, 3, 4.0, 5, 6.0, 0));
+  ASSERT(21, sum2(1, 2.0, 3, 4.0, 5, 6.0, 0));
+  ASSERT(210, sum2(1, 2.0, 3, 4.0, 5, 6.0, 7, 8.0, 9, 10.0, 11, 12.0, 13, 14.0,
+                   15, 16.0, 17, 18.0, 19, 20.0, 0));
+  ASSERT(0, ({
+           char buf[100];
+           fmt(buf, "%d %d", 2, 3);
+           strcmp(buf, "2 3");
+         }));
+
+  return 0;
+}