sys_stat: implement utimensat() POSIX function

libstdc++ requires utimensat for file timestamp operations. Implemented
using openat(O_PATH) + futimens, with AT_SYMLINK_NOFOLLOW support.

src/header/sys_stat/mod.rs

@@ -142,6 +142,39 @@ pub unsafe extern "C" fn futimens(fd: c_int, times: *const timespec) -> c_int {
         .or_minus_one_errno()
 }
 
+/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/utimensat.html>.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn utimensat(
+    dirfd: c_int,
+    path: *const c_char,
+    times: *const timespec,
+    flags: c_int,
+) -> c_int {
+    // If path is NULL, behave like futimens.
+    if path.is_null() {
+        return unsafe { futimens(dirfd, times) };
+    }
+
+    let path = unsafe { CStr::from_ptr(path) };
+
+    let mut oflag = O_PATH;
+    if flags & crate::header::fcntl::AT_SYMLINK_NOFOLLOW != 0 {
+        oflag |= O_NOFOLLOW;
+    }
+
+    let fd = Sys::openat(dirfd, path, oflag, 0).or_minus_one_errno();
+    if fd < 0 {
+        return -1;
+    }
+
+    let res = unsafe { Sys::futimens(fd, times) }
+        .map(|()| 0)
+        .or_minus_one_errno();
+
+    let _ = Sys::close(fd);
+    res
+}
+
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/lstat.html>.
 #[unsafe(no_mangle)]
 pub unsafe extern "C" fn lstat(path: *const c_char, buf: *mut stat) -> c_int {