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.

tool/viz/fold.c

@@ -29,7 +29,7 @@
 
 int column_;
 
-noreturn void usage(int rc, FILE *f) {
+wontreturn void usage(int rc, FILE *f) {
   fputs("Usage: ", f);
   fputs(program_invocation_name, f);
   fputs(" [-w COLS] [FILE...]\n", f);