std: Fix build for NuttX targets

Fix std build for all NuttX targets. It is the single largest set of
failures on <https://does-it-build.noratrieb.dev/>. Although, ESP-IDF
also requires these same gates, there are other issues for those
targets.

This can verified be running `x check library/std --target=` for all
NuttX targets.

library/std/src/sys/fd/unix.rs

@@ -71,9 +71,11 @@ const fn max_iov() -> usize {
     target_os = "android",
     target_os = "dragonfly",
     target_os = "emscripten",
+    target_os = "espidf",
     target_os = "freebsd",
     target_os = "linux",
     target_os = "netbsd",
+    target_os = "nuttx",
     target_os = "nto",
     target_os = "openbsd",
     target_os = "horizon",

library/std/src/sys/net/connection/socket/unix.rs

@@ -1,5 +1,6 @@
 use libc::{MSG_PEEK, c_int, c_void, size_t, sockaddr, socklen_t};
 
+#[cfg(not(any(target_os = "espidf", target_os = "nuttx")))]
 use crate::ffi::CStr;
 use crate::io::{self, BorrowedBuf, BorrowedCursor, IoSlice, IoSliceMut};
 use crate::net::{Shutdown, SocketAddr};