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

tool/viz/cpuid.c

@@ -175,6 +175,8 @@ int main(int argc, char *argv[]) {
              : "");
 
   CANIUSE(AVXVNNI);
+  CANIUSE(AVXVNNIINT8);
+  CANIUSE(AVXVNNIINT16);
   CANIUSE(AVX512BW);
   CANIUSE(AVX512CD);
   CANIUSE(AVX512DQ);