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/strarray.c

@@ -5,12 +5,10 @@ void strarray_push(StringArray *arr, char *s) {
     arr->data = calloc(8, sizeof(char *));
     arr->capacity = 8;
   }
-
   if (arr->capacity == arr->len) {
     arr->data = realloc(arr->data, sizeof(char *) * arr->capacity * 2);
     arr->capacity *= 2;
     for (int i = arr->len; i < arr->capacity; i++) arr->data[i] = NULL;
   }
-
   arr->data[arr->len++] = s;
 }