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.

libc/str/strspn.c

@@ -27,7 +27,7 @@
  * @see strcspn(), strtok_r()
  * @asyncsignalsafe
  */
-size_t(strspn)(const char *s, const char *accept) {
+size_t strspn(const char *s, const char *accept) {
   size_t i;
   for (i = 0; s[i]; ++i) {
     if (!HasCharacter(s[i], accept)) {