Rollup merge of #124480 - Enselic:on-broken-pipe, r=jieyouxu

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

In the stabilization [attempt](rust-lang/rust#120832) of `#[unix_sigpipe = "sig_dfl"]`, a concern was [raised ](rust-lang/rust#120832 (comment)) 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 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](rust-lang/rust#120832 (comment)), 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.

Tracking issue: rust-lang/rust#97889

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(