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/usualconv_test.c

@@ -0,0 +1,58 @@
+#include "third_party/chibicc/test/test.h"
+
+static int ret10(void) {
+  return 10;
+}
+
+int main() {
+  ASSERT((long)-5, -10 + (long)5);
+  ASSERT((long)-15, -10 - (long)5);
+  ASSERT((long)-50, -10 * (long)5);
+  ASSERT((long)-2, -10 / (long)5);
+
+  ASSERT(1, -2 < (long)-1);
+  ASSERT(1, -2 <= (long)-1);
+  ASSERT(0, -2 > (long)-1);
+  ASSERT(0, -2 >= (long)-1);
+
+  ASSERT(1, (long)-2 < -1);
+  ASSERT(1, (long)-2 <= -1);
+  ASSERT(0, (long)-2 > -1);
+  ASSERT(0, (long)-2 >= -1);
+
+  ASSERT(0, 2147483647 + 2147483647 + 2);
+  ASSERT((long)-1, ({
+           long x;
+           x = -1;
+           x;
+         }));
+
+  ASSERT(1, ({
+           char x[3];
+           x[0] = 0;
+           x[1] = 1;
+           x[2] = 2;
+           char *y = x + 1;
+           y[0];
+         }));
+  ASSERT(0, ({
+           char x[3];
+           x[0] = 0;
+           x[1] = 1;
+           x[2] = 2;
+           char *y = x + 1;
+           y[-1];
+         }));
+  ASSERT(5, ({
+           struct t {
+             char a;
+           } x, y;
+           x.a = 5;
+           y = x;
+           y.a;
+         }));
+
+  ASSERT(10, (1 ? ret10 : (void *)0)());
+
+  return 0;
+}