Conditionally use asm!

This avoids a compiler warning on single-core targets which do not
perform an actual check. This fixes esp-rs#36.

Author: sirhcel

src/cpu.rs

@@ -1,3 +1,4 @@
+#[cfg(any(esp32, esp32s3))]
 use core::arch::asm;
 
 use esp_idf_sys::*;