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/vla_test.c

@@ -0,0 +1,86 @@
+#include "third_party/chibicc/test/test.h"
+
+int main() {
+  ASSERT(20, ({
+           int n = 5;
+           int x[n];
+           sizeof(x);
+         }));
+  ASSERT((5 + 1) * (8 * 2) * 4, ({
+           int m = 5, n = 8;
+           int x[m + 1][n * 2];
+           sizeof(x);
+         }));
+
+  ASSERT(8, ({
+           char n = 10;
+           int(*x)[n][n + 2];
+           sizeof(x);
+         }));
+  ASSERT(480, ({
+           char n = 10;
+           int(*x)[n][n + 2];
+           sizeof(*x);
+         }));
+  ASSERT(48, ({
+           char n = 10;
+           int(*x)[n][n + 2];
+           sizeof(**x);
+         }));
+  ASSERT(4, ({
+           char n = 10;
+           int(*x)[n][n + 2];
+           sizeof(***x);
+         }));
+
+  ASSERT(60, ({
+           char n = 3;
+           int x[5][n];
+           sizeof(x);
+         }));
+  ASSERT(12, ({
+           char n = 3;
+           int x[5][n];
+           sizeof(*x);
+         }));
+
+  ASSERT(60, ({
+           char n = 3;
+           int x[n][5];
+           sizeof(x);
+         }));
+  ASSERT(20, ({
+           char n = 3;
+           int x[n][5];
+           sizeof(*x);
+         }));
+
+  ASSERT(0, ({
+           int n = 10;
+           int x[n + 1][n + 6];
+           int *p = x;
+           for (int i = 0; i < sizeof(x) / 4; i++) p[i] = i;
+           x[0][0];
+         }));
+  ASSERT(5, ({
+           int n = 10;
+           int x[n + 1][n + 6];
+           int *p = x;
+           for (int i = 0; i < sizeof(x) / 4; i++) p[i] = i;
+           x[0][5];
+         }));
+  ASSERT(5 * 16 + 2, ({
+           int n = 10;
+           int x[n + 1][n + 6];
+           int *p = x;
+           for (int i = 0; i < sizeof(x) / 4; i++) p[i] = i;
+           x[5][2];
+         }));
+
+  ASSERT(10, ({
+           int n = 5;
+           sizeof(char[2][n]);
+         }));
+
+  return 0;
+}