perf(uucore): call rt_sigaction once not 62 times

`./coreutils true` becomes 6% faster. For some unknown reason,
`capture_startup_state` runs 62 times, so I'll just hotfix it to
immediately return from the second time onwards.

When 2d7a3bf2e0 introduced the fcntl
calls, each group of 3 calls was repeated 4 times. Some time later, it
became 62 times. Now it runs only 1 time.

src/uucore/src/lib/features/signals.rs

@@ -538,6 +538,9 @@ static STDERR_WAS_CLOSED: AtomicBool = AtomicBool::new(false);
 #[cfg(unix)]
 static SIGPIPE_WAS_IGNORED: AtomicBool = AtomicBool::new(false);
 
+#[cfg(unix)]
+static STARTUP_STATE_WAS_CAPTURED: AtomicBool = AtomicBool::new(false);
+
 /// Captures stdio and SIGPIPE state at process initialization, before main() runs.
 ///
 /// # Safety
@@ -549,6 +552,11 @@ pub unsafe extern "C" fn capture_startup_state() {
     use std::mem::MaybeUninit;
     use std::ptr;
 
+    // No spinlock because we're single-threaded at this point
+    if STARTUP_STATE_WAS_CAPTURED.swap(true, Ordering::Relaxed) {
+        return;
+    }
+
     // Capture stdio state
     unsafe {
         STDIN_WAS_CLOSED.store(