Fix breakages in Linux-only build modes

- compile.com now polyfills -march=native which gcc/clang removed
- Guarantee zero Windows code is linked into non-Windows binaries
- MODE=tinylinux binaries are now back to being as tiny as ~4kb
- Improve the runtime's stack allocation / alignment hack
- GitHub Actions now tests Linux modes for assurance

third_party/lua/lunix.c

@@ -205,7 +205,7 @@ int LuaUnixSysretErrno(lua_State *L, const char *call, int olderr) {
   struct UnixErrno *ep;
   int i, unixerr, winerr;
   unixerr = errno;
-  winerr = GetLastError();
+  winerr = IsWindows() ? GetLastError() : 0;
   if (!IsTiny() && !(0 < unixerr && unixerr < (!IsWindows() ? 4096 : 65536))) {
     WARNF("errno should not be %d", unixerr);
   }