executor: arch-std: Make Signaler public.

When you have the feature = 'arch-std' enabled.
Then the executor code creates the function '__pender()'.
But this function used `Signaler` internally.
The problem is that `Signaler` is currently private.

Make it public so that it can be used to create one for the
`raw::executor::new()`.

embassy-executor/src/arch/std.rs

@@ -64,13 +64,13 @@ mod thread {
         }
     }
 
-    struct Signaler {
+    pub struct Signaler {
         mutex: Mutex<bool>,
         condvar: Condvar,
     }
 
     impl Signaler {
-        fn new() -> Self {
+        pub fn new() -> Self {
             Self {
                 mutex: Mutex::new(false),
                 condvar: Condvar::new(),