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/stdio/g_stdin.c

@@ -24,7 +24,7 @@ STATIC_YOINK("_init_g_stdin");
 
 FILE *stdin;
 hidden FILE g_stdin;
-hidden unsigned char g_stdin_buf[BUFSIZ] aligned(PAGESIZE);
+hidden unsigned char g_stdin_buf[BUFSIZ] forcealign(PAGESIZE);
 
 static textstartup void g_stdin_init() {
   _fflushregister(stdin);