Make sentinel value configurable

There are OSs that always return the lowest free value.
The algorithm in `lazy_init` always avoids keys with the
sentinel value.
In affected OSs, this means that each call to `lazy_init`
will always request two keys from the OS and returns/frees
the first one (with sentinel value) immediately afterwards.

By making the sentinel value configurable, affected OSs can
use a different value than zero to prevent this performance
issue.

library/std/src/sys_common/thread_local_key.rs

@@ -117,10 +117,14 @@ pub struct Key {
 /// This value specifies no destructor by default.
 pub const INIT: StaticKey = StaticKey::new(None);
 
+// Define a sentinel value that is unlikely to be returned
+// as a TLS key (but it may be returned).
+const KEY_SENTVAL: usize = 0;
+
 impl StaticKey {
     #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
     pub const fn new(dtor: Option<unsafe extern "C" fn(*mut u8)>) -> StaticKey {
-        StaticKey { key: atomic::AtomicUsize::new(0), dtor }
+        StaticKey { key: atomic::AtomicUsize::new(KEY_SENTVAL), dtor }
     }
 
     /// Gets the value associated with this TLS key
@@ -144,31 +148,36 @@ impl StaticKey {
     #[inline]
     unsafe fn key(&self) -> imp::Key {
         match self.key.load(Ordering::Relaxed) {
-            0 => self.lazy_init() as imp::Key,
+            KEY_SENTVAL => self.lazy_init() as imp::Key,
             n => n as imp::Key,
         }
     }
 
     unsafe fn lazy_init(&self) -> usize {
-        // POSIX allows the key created here to be 0, but the compare_exchange
-        // below relies on using 0 as a sentinel value to check who won the
+        // POSIX allows the key created here to be KEY_SENTVAL, but the compare_exchange
+        // below relies on using KEY_SENTVAL as a sentinel value to check who won the
         // race to set the shared TLS key. As far as I know, there is no
         // guaranteed value that cannot be returned as a posix_key_create key,
         // so there is no value we can initialize the inner key with to
         // prove that it has not yet been set. As such, we'll continue using a
-        // value of 0, but with some gyrations to make sure we have a non-0
+        // value of KEY_SENTVAL, but with some gyrations to make sure we have a non-KEY_SENTVAL
         // value returned from the creation routine.
         // FIXME: this is clearly a hack, and should be cleaned up.
         let key1 = imp::create(self.dtor);
-        let key = if key1 != 0 {
+        let key = if key1 as usize != KEY_SENTVAL {
             key1
         } else {
             let key2 = imp::create(self.dtor);
             imp::destroy(key1);
             key2
         };
-        rtassert!(key != 0);
-        match self.key.compare_exchange(0, key as usize, Ordering::SeqCst, Ordering::SeqCst) {
+        rtassert!(key as usize != KEY_SENTVAL);
+        match self.key.compare_exchange(
+            KEY_SENTVAL,
+            key as usize,
+            Ordering::SeqCst,
+            Ordering::SeqCst,
+        ) {
             // The CAS succeeded, so we've created the actual key
             Ok(_) => key as usize,
             // If someone beat us to the punch, use their key instead