rb: remove FEXEC_STEP reference (removed from redox_rt)

Commit bb20fe9c in relibc fork removed FEXEC_STEP diagnostic from
redox_rt::proc. The bootstrap binary in base referenced it in
exec.rs:257, causing build failure:
  error[E0425]: cannot find value  in module

Removed the diagnostic step variable and simplified the panic message.
Build now compiles cleanly.

bootstrap/src/exec.rs

@@ -254,8 +254,7 @@ pub fn main() -> ! {
         None,
     )
     .unwrap_or_else(|e| {
-        let step = unsafe { redox_rt::proc::FEXEC_STEP };
-        panic!("fexec_impl error: {:?} at step {}", e, step);
+        panic!("fexec_impl error: {:?}", e);
     })
     .unwrap_or_else(|| panic!("fexec_impl returned None (no interpreter)"))
     else {