Replace unidiomatic 'extern crate' to 'use x as _'

examples/test_on_host.rs

@@ -23,10 +23,10 @@
 
 // pick a panicking behavior
 #[cfg(not(test))]
-extern crate panic_halt; // you can put a breakpoint on `rust_begin_unwind` to catch panics
-// extern crate panic_abort; // requires nightly
-// extern crate panic_itm; // logs messages over ITM; requires ITM support
-// extern crate panic_semihosting; // logs messages to the host stderr; requires a debugger
+use panic_halt as _; // you can put a breakpoint on `rust_begin_unwind` to catch panics
+// use panic_abort as _; // requires nightly
+// use panic_itm as _; // logs messages over ITM; requires ITM support
+// use panic_semihosting as _; // logs messages to the host stderr; requires a debugger
 
 use cortex_m::asm;
 use cortex_m_rt::entry;