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_codegen_llvm/src/back/write.rs

@@ -152,7 +152,8 @@ pub fn target_machine_factory(
     let features = features.join(",");
     let features = CString::new(features).unwrap();
     let abi = SmallCStr::new(&sess.target.llvm_abiname);
-    let trap_unreachable = sess.target.trap_unreachable;
+    let trap_unreachable =
+        sess.opts.debugging_opts.trap_unreachable.unwrap_or(sess.target.trap_unreachable);
     let emit_stack_size_section = sess.opts.debugging_opts.emit_stack_sizes;
 
     let asm_comments = sess.asm_comments();

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));

compiler/rustc_session/src/options.rs

@@ -1113,6 +1113,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "choose the TLS model to use (`rustc --print tls-models` for details)"),
     trace_macros: bool = (false, parse_bool, [UNTRACKED],
         "for every macro invocation, print its name and arguments (default: no)"),
+    trap_unreachable: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "generate trap instructions for unreachable intrinsics (default: use target setting, usually yes)"),
     treat_err_as_bug: Option<usize> = (None, parse_treat_err_as_bug, [TRACKED],
         "treat error number `val` that occurs as bug"),
     trim_diagnostic_paths: bool = (true, parse_bool, [UNTRACKED],