Make improvements

- Let OpenMP be usable via cosmocc
- Let libunwind be usable via cosmocc
- Make X86_HAVE(AVXVNNI) work correctly
- Avoid using MAP_GROWSDOWN on qemu-aarch64
- Introduce in6addr_any and in6addr_loopback
- Have thread stacks use MAP_GROWSDOWN by default
- Ask OpenMP to not use filesystem to manage threads
- Make NI_MAXHOST and NI_MAXSERV available w/o _GNU_SOURCE

libc/intrin/getmainstack.c

@@ -103,7 +103,7 @@ void __get_main_stack(void **out_addr, size_t *out_size, int *out_guardsize) {
   if (IsWindows()) {
     *out_addr = (void *)GetStaticStackAddr(0);
     *out_size = GetStaticStackSize();
-    *out_guardsize = GetGuardSize();
+    *out_guardsize = getauxval(AT_PAGESZ);
     return;
   }
   int pagesz = getauxval(AT_PAGESZ);