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.

compiler/rustc_passes/src/errors.rs

@@ -1259,13 +1259,6 @@ pub struct ExternMain {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(passes_unix_sigpipe_values)]
-pub struct UnixSigpipeValues {
-    #[primary_span]
-    pub span: Span,
-}
-
 pub struct NoMainErr {
     pub sp: Span,
     pub crate_name: Symbol,