Use the sym operator for inline assembly (#5459)

* Use the `sym` operator for inline assembly

Avoids extra `#[no_mangle]` functions and undue symbols being exposed
from Wasmtime. This is a newly stabilized feature in Rust 1.66.0. I've
also added a `rust-version` entry to the `wasmtime` crate to try to head
off possible reports in the future about odd error messages or usage of
unstable features if the rustc version is too old.

* Fix a s390x warning

* Add `rust-version` annotation to Wasmtime crate

As the other main entrypoint for embedders.

crates/fiber/src/unix.rs

@@ -110,6 +110,8 @@ extern "C" {
         entry_arg0: *mut u8,
     );
     fn wasmtime_fiber_switch(top_of_stack: *mut u8);
+    #[allow(dead_code)] // only used in inline assembly for some platforms
+    fn wasmtime_fiber_start();
 }
 
 extern "C" fn fiber_start<F, A, B, C>(arg0: *mut u8, top_of_stack: *mut u8)