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/constexpr_test.c

@@ -0,0 +1,181 @@
+#include "third_party/chibicc/test/test.h"
+
+float g40 = 1.5;
+double g41 = 0.0 ? 55 : (0, 1 + 1 * 5.0 / 2 * (double)2 * (int)2.0);
+
+int main() {
+  ASSERT(10, ({
+           enum { ten = 1 + 2 + 3 + 4 };
+           ten;
+         }));
+  ASSERT(1, ({
+           int i = 0;
+           switch (3) {
+             case 5 - 2 + 0 * 3:
+               i++;
+           }
+           i;
+         }));
+  ASSERT(8, ({
+           int x[1 + 1];
+           sizeof(x);
+         }));
+  ASSERT(6, ({
+           char x[8 - 2];
+           sizeof(x);
+         }));
+  ASSERT(6, ({
+           char x[2 * 3];
+           sizeof(x);
+         }));
+  ASSERT(3, ({
+           char x[12 / 4];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[12 % 10];
+           sizeof(x);
+         }));
+  ASSERT(0b100, ({
+           char x[0b110 & 0b101];
+           sizeof(x);
+         }));
+  ASSERT(0b111, ({
+           char x[0b110 | 0b101];
+           sizeof(x);
+         }));
+  ASSERT(0b110, ({
+           char x[0b111 ^ 0b001];
+           sizeof(x);
+         }));
+  ASSERT(4, ({
+           char x[1 << 2];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[4 >> 1];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[(1 == 1) + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(1 != 1) + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(1 < 1) + 1];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[(1 <= 1) + 1];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[1 ? 2 : 3];
+           sizeof(x);
+         }));
+  ASSERT(3, ({
+           char x[0 ? 2 : 3];
+           sizeof(x);
+         }));
+  ASSERT(3, ({
+           char x[(1, 3)];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[!0 + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[!1 + 1];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[~-3];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[(5 || 6) + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(0 || 0) + 1];
+           sizeof(x);
+         }));
+  ASSERT(2, ({
+           char x[(1 && 1) + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(1 && 0) + 1];
+           sizeof(x);
+         }));
+  ASSERT(3, ({
+           char x[(int)3];
+           sizeof(x);
+         }));
+  ASSERT(15, ({
+           char x[(char)0xffffff0f];
+           sizeof(x);
+         }));
+  ASSERT(0x10f, ({
+           char x[(short)0xffff010f];
+           sizeof(x);
+         }));
+  ASSERT(4, ({
+           char x[(int)0xfffffffffff + 5];
+           sizeof(x);
+         }));
+  ASSERT(8, ({
+           char x[(int*)0 + 2];
+           sizeof(x);
+         }));
+  ASSERT(12, ({
+           char x[(int*)16 - 1];
+           sizeof(x);
+         }));
+  ASSERT(3, ({
+           char x[(int*)16 - (int*)4];
+           sizeof(x);
+         }));
+
+  ASSERT(4, ({
+           char x[(-1 >> 31) + 5];
+           sizeof(x);
+         }));
+  ASSERT(255, ({
+           char x[(unsigned char)0xffffffff];
+           sizeof(x);
+         }));
+  ASSERT(0x800f, ({
+           char x[(unsigned short)0xffff800f];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(unsigned int)0xfffffffffff >> 31];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(long)-1 / ((long)1 << 62) + 1];
+           sizeof(x);
+         }));
+  ASSERT(4, ({
+           char x[(unsigned long)-1 / ((long)1 << 62) + 1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(unsigned)1 < -1];
+           sizeof(x);
+         }));
+  ASSERT(1, ({
+           char x[(unsigned)1 <= -1];
+           sizeof(x);
+         }));
+
+  ASSERT(1, g40 == 1.5);
+  ASSERT(1, g41 == 11);
+
+  return 0;
+}