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/decl_test.c

@@ -0,0 +1,51 @@
+#include "third_party/chibicc/test/test.h"
+
+int main() {
+  ASSERT(1, ({
+           char x;
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           short int x;
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           int short x;
+           sizeof(x);
+         }));
+  ASSERT(4, ({
+           int x;
+           sizeof(x);
+         }));
+  ASSERT(8, ({
+           long int x;
+           sizeof(x);
+         }));
+  ASSERT(8, ({
+           int long x;
+           sizeof(x);
+         }));
+
+  ASSERT(8, ({
+           long long x;
+           sizeof(x);
+         }));
+
+  ASSERT(0, ({
+           _Bool x = 0;
+           x;
+         }));
+  ASSERT(1, ({
+           _Bool x = 1;
+           x;
+         }));
+  ASSERT(1, ({
+           _Bool x = 2;
+           x;
+         }));
+  ASSERT(1, (_Bool)1);
+  ASSERT(1, (_Bool)2);
+  ASSERT(0, (_Bool)(char)256);
+
+  return 0;
+}