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_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],