Use cfg!() instead of #[cfg] for most profiling code

This makes it less likely that it breaks again in the future as most
code gets checked by the compiler even when profiling support is not
enabled.

src/startup/mod.rs

@@ -198,7 +198,6 @@ pub(crate) fn kmain_ap(cpu_id: crate::cpu_set::LogicalCpuId) -> ! {
         hint::spin_loop();
     }
 
-    #[cfg(feature = "profiling")]
     profiling::maybe_run_profiling_helper_forever(cpu_id);
 
     if !cfg!(feature = "multi_core") {