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/alloca_test.c

@@ -0,0 +1,29 @@
+#include "third_party/chibicc/test/test.h"
+
+void *fn(int x, void *p, int y) {
+  return p;
+}
+
+int main() {
+  int i = 0;
+
+  char *p1 = alloca(16);
+  char *p2 = alloca(16);
+  char *p3 = 1 + (char *)alloca(3) + 1;
+  p3 -= 2;
+  char *p4 = fn(1, alloca(16), 3);
+
+  ASSERT(16, p1 - p2);
+  ASSERT(16, p2 - p3);
+  ASSERT(16, p3 - p4);
+
+  memcpy(p1, "0123456789abcdef", 16);
+  memcpy(p2, "ghijklmnopqrstuv", 16);
+  memcpy(p3, "wxy", 3);
+
+  ASSERT(0, memcmp(p1, "0123456789abcdef", 16));
+  ASSERT(0, memcmp(p2, "ghijklmnopqrstuv", 16));
+  ASSERT(0, memcmp(p3, "wxy", 3));
+
+  return 0;
+}