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.

library/std/src/rt.rs

@@ -74,7 +74,7 @@ macro_rules! rtunwrap {
 //
 // Since 2014, the Rust runtime on Unix has set the `SIGPIPE` handler to
 // `SIG_IGN`. Applications have good reasons to want a different behavior
-// though, so there is a `#[unix_sigpipe = "..."]` attribute on `fn main()` that
+// though, so there is a `-Zon-broken-pipe` compiler flag that
 // can be used to select how `SIGPIPE` shall be setup (if changed at all) before
 // `fn main()` is called. See <https://github.com/rust-lang/rust/issues/97889>
 // for more info.