executor: add missing main macro reexport on xtensa.

embassy-executor/src/arch/xtensa.rs

@@ -8,6 +8,9 @@ mod thread {
     use core::marker::PhantomData;
     use core::sync::atomic::{AtomicBool, Ordering};
 
+    #[cfg(feature = "nightly")]
+    pub use embassy_macros::main_riscv as main;
+
     use crate::{raw, Spawner};
 
     /// global atomic used to keep track of whether there is work to do since sev() is not available on Xtensa