Fix bugs in cosmocc toolchain

This change integrates e58abc1110b335a3341e8ad5821ad8e3880d9bb2 from
https://github.com/ahgamut/musl-cross-make/ which fixes the issues we
were having with our C language extension for symbolic constants. This
change also performs some code cleanup and bug fixes to getaddrinfo().
It's now possible to compile projects like ncurses, readline and python
without needing to patch anything upstream, except maybe a line or two.
Pretty soon it should be possible to build a Linux distro on Cosmo.

third_party/lua/lapi.c

@@ -1524,7 +1524,7 @@ LUA_API int lua_error (lua_State *L) {
   else
     luaG_errormsg(L);  /* raise a regular error */
   /* code unreachable; will unlock when control actually leaves the kernel */
-  unreachable;
+  __builtin_unreachable();
 }