Allow disabling TrapUnreachable via -Ztrap-unreachable=no

This is useful for embedded targets where small code size is desired.
For example, on my project (thumbv7em-none-eabi) this yields a 0.6% code size reduction.

compiler/rustc_interface/src/tests.rs

@@ -592,6 +592,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(thinlto, Some(true));
     tracked!(tune_cpu, Some(String::from("abc")));
     tracked!(tls_model, Some(TlsModel::GeneralDynamic));
+    tracked!(trap_unreachable, Some(false));
     tracked!(treat_err_as_bug, Some(1));
     tracked!(unleash_the_miri_inside_of_you, true);
     tracked!(use_ctors_section, Some(true));