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/net/echoserver.c

@@ -100,7 +100,7 @@ nodiscard char *DescribeSocket(struct Socket *s) {
                    gc(DescribeAddress(&s->addr)));
 }
 
-noreturn void ShowUsageAndExit(bool iserror) {
+wontreturn void ShowUsageAndExit(bool iserror) {
   FILE *f = iserror ? stderr : stdout;
   int rc = iserror ? EXIT_FAILURE : EXIT_SUCCESS;
   fprintf(f, "%s: %s %s\n", "Usage", g_argv[0], "PROTOCOL:ADDR:PORT...");