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_target/src/spec/mod.rs

@@ -778,6 +778,14 @@ pub enum PanicStrategy {
     Abort,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Hash, Encodable, Decodable, HashStable_Generic)]
+pub enum OnBrokenPipe {
+    Default,
+    Kill,
+    Error,
+    Inherit,
+}
+
 impl PanicStrategy {
     pub fn desc(&self) -> &str {
         match *self {