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/alignof_test.c

@@ -0,0 +1,103 @@
+#include "third_party/chibicc/test/test.h"
+
+int _Alignas(512) g1;
+int _Alignas(512) g2;
+char g3;
+int g4;
+long g5;
+char g6;
+
+int main() {
+  ASSERT(1, _Alignof(char));
+  ASSERT(2, _Alignof(short));
+  ASSERT(4, _Alignof(int));
+  ASSERT(8, _Alignof(long));
+  ASSERT(8, _Alignof(long long));
+  ASSERT(1, _Alignof(char[3]));
+  ASSERT(4, _Alignof(int[3]));
+  ASSERT(1, _Alignof(struct {
+           char a;
+           char b;
+         }[2]));
+  ASSERT(8, _Alignof(struct {
+           char a;
+           long b;
+         }[2]));
+
+  ASSERT(1, ({
+           _Alignas(char) char x, y;
+           &y - &x;
+         }));
+  ASSERT(8, ({
+           _Alignas(long) char x, y;
+           &y - &x;
+         }));
+  ASSERT(32, ({
+           _Alignas(32) char x, y;
+           &y - &x;
+         }));
+  ASSERT(32, ({
+           _Alignas(32) int *x, *y;
+           ((char *)&y) - ((char *)&x);
+         }));
+  ASSERT(16, ({
+           struct {
+             _Alignas(16) char x, y;
+           } a;
+           &a.y - &a.x;
+         }));
+  ASSERT(8, ({
+           struct T {
+             _Alignas(8) char a;
+           };
+           _Alignof(struct T);
+         }));
+
+  ASSERT(0, (long)(char *)&g1 % 512);
+  ASSERT(0, (long)(char *)&g2 % 512);
+  ASSERT(0, (long)(char *)&g4 % 4);
+  ASSERT(0, (long)(char *)&g5 % 8);
+
+  ASSERT(1, ({
+           char x;
+           _Alignof(x);
+         }));
+  ASSERT(4, ({
+           int x;
+           _Alignof(x);
+         }));
+  ASSERT(1, ({
+           char x;
+           _Alignof x;
+         }));
+  ASSERT(4, ({
+           int x;
+           _Alignof x;
+         }));
+
+  ASSERT(1, _Alignof(char) << 31 >> 31);
+  ASSERT(1, _Alignof(char) << 63 >> 63);
+  ASSERT(1, ({
+           char x;
+           _Alignof(x) << 63 >> 63;
+         }));
+
+  ASSERT(0, ({
+           char x[16];
+           (unsigned long)&x % 16;
+         }));
+  ASSERT(0, ({
+           char x[17];
+           (unsigned long)&x % 16;
+         }));
+  ASSERT(0, ({
+           char x[100];
+           (unsigned long)&x % 16;
+         }));
+  ASSERT(0, ({
+           char x[101];
+           (unsigned long)&x % 16;
+         }));
+
+  return 0;
+}