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/messages.ftl

@@ -695,9 +695,6 @@ passes_transparent_incompatible =
 passes_undefined_naked_function_abi =
     Rust ABI is unsupported in naked functions
 
-passes_unix_sigpipe_values =
-    valid values for `#[unix_sigpipe = "..."]` are `inherit`, `sig_ign`, or `sig_dfl`
-
 passes_unknown_external_lang_item =
     unknown external lang item: `{$lang_item}`