Change SIGPIPE ui from #[unix_sigpipe = "..."] to -Zon-broken-pipe=...

In the stabilization attempt of `#[unix_sigpipe = "sig_dfl"]`, a concern
was raised related to using a language attribute for the feature: Long
term, we want `fn lang_start()` to be definable by any crate, not just
libstd. Having a special language attribute in that case becomes
awkward.

So as a first step towards towards the next stabilization attempt, this
PR changes the `#[unix_sigpipe = "..."]` attribute to a compiler flag
`-Zon-broken-pipe=...` to remove that concern, since now the language
is not "contaminated" by this feature.

Another point was also raised, namely that the ui should not leak
**how** it does things, but rather what the **end effect** is. The new
flag uses the proposed naming. This is of course something that can be
iterated on further before stabilization.

Author: Enselic

src/eval.rs

@@ -387,7 +387,7 @@ pub fn create_ecx<'mir, 'tcx: 'mir>(
             let main_ptr = ecx.fn_ptr(FnVal::Instance(entry_instance));
 
             // Always using DEFAULT is okay since we don't support signals in Miri anyway.
-            // (This means we are effectively ignoring `#[unix_sigpipe]`.)
+            // (This means we are effectively ignoring `-Zon-broken-pipe`.)
             let sigpipe = rustc_session::config::sigpipe::DEFAULT;
 
             ecx.call_function(