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_session/src/config.rs

@@ -2891,7 +2891,9 @@ pub(crate) mod dep_tracking {
     use rustc_feature::UnstableFeatures;
     use rustc_span::edition::Edition;
     use rustc_span::RealFileName;
-    use rustc_target::spec::{CodeModel, MergeFunctions, PanicStrategy, RelocModel, WasmCAbi};
+    use rustc_target::spec::{
+        CodeModel, MergeFunctions, OnBrokenPipe, PanicStrategy, RelocModel, WasmCAbi,
+    };
     use rustc_target::spec::{
         RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TargetTriple, TlsModel,
     };
@@ -2955,6 +2957,7 @@ pub(crate) mod dep_tracking {
         InstrumentXRay,
         CrateType,
         MergeFunctions,
+        OnBrokenPipe,
         PanicStrategy,
         RelroLevel,
         OptLevel,