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/bitfield_test.c

@@ -0,0 +1,128 @@
+#include "third_party/chibicc/test/test.h"
+
+struct {
+  char a;
+  int b : 5;
+  int c : 10;
+} g45 = {1, 2, 3}, g46 = {};
+
+int main() {
+  ASSERT(4, sizeof(struct { int x : 1; }));
+  ASSERT(8, sizeof(struct { long x : 1; }));
+
+  struct bit1 {
+    short a;
+    char b;
+    int c : 2;
+    int d : 3;
+    int e : 3;
+  };
+
+  ASSERT(4, sizeof(struct bit1));
+  ASSERT(1, ({
+           struct bit1 x;
+           x.a = 1;
+           x.b = 2;
+           x.c = 3;
+           x.d = 4;
+           x.e = 5;
+           x.a;
+         }));
+  ASSERT(1, ({
+           struct bit1 x = {1, 2, 3, 4, 5};
+           x.a;
+         }));
+  ASSERT(2, ({
+           struct bit1 x = {1, 2, 3, 4, 5};
+           x.b;
+         }));
+  ASSERT(-1, ({
+    struct bit1 x = {1, 2, 3, 4, 5};
+    x.c;
+  }));
+  ASSERT(-4, ({
+    struct bit1 x = {1, 2, 3, 4, 5};
+    x.d;
+  }));
+  ASSERT(-3, ({
+    struct bit1 x = {1, 2, 3, 4, 5};
+    x.e;
+  }));
+
+  ASSERT(1, g45.a);
+  ASSERT(2, g45.b);
+  ASSERT(3, g45.c);
+
+  ASSERT(0, g46.a);
+  ASSERT(0, g46.b);
+  ASSERT(0, g46.c);
+
+  typedef struct {
+    int a : 10;
+    int b : 10;
+    int c : 10;
+  } T3;
+
+  ASSERT(1, ({
+           T3 x = {1, 2, 3};
+           x.a++;
+         }));
+  ASSERT(2, ({
+           T3 x = {1, 2, 3};
+           x.b++;
+         }));
+  ASSERT(3, ({
+           T3 x = {1, 2, 3};
+           x.c++;
+         }));
+
+  ASSERT(2, ({
+           T3 x = {1, 2, 3};
+           ++x.a;
+         }));
+  ASSERT(3, ({
+           T3 x = {1, 2, 3};
+           ++x.b;
+         }));
+  ASSERT(4, ({
+           T3 x = {1, 2, 3};
+           ++x.c;
+         }));
+
+  ASSERT(4, sizeof(struct {
+           int a : 3;
+           int c : 1;
+           int c : 5;
+         }));
+  ASSERT(8, sizeof(struct {
+           int a : 3;
+           int : 0;
+           int c : 5;
+         }));
+  ASSERT(4, sizeof(struct {
+           int a : 3;
+           int : 0;
+         }));
+
+  typedef struct {
+    long p : 1;
+    long a : 40;
+    long b : 20;
+  } T4;
+
+  ASSERT(0xfffffffffffaaaaa, ({
+           T4 x = {1, 0x31337313371337, 0xaaaaaaaaaaaaaaaa};
+           x.b++;
+         }));
+  ASSERT(0x7313371337, ({
+           T4 x = {1, 0x31337313371337, 0xaaaaaaaaaaaaaaaa};
+           x.a++;
+         }));
+  ASSERT(0xffffffffffffffd4, ({
+           T4 x;
+           x.b = -44;
+           x.b++;
+         }));
+
+  return 0;
+}