Support #[unix_sigpipe = "inherit|sig_dfl|sig_ign"] on fn main()

This makes it possible to instruct libstd to never touch the signal
handler for `SIGPIPE`, which makes programs pipeable by default (e.g.
with `./your-program | head -n 1`) without `ErrorKind::BrokenPipe`
errors.

src/test/ui/attributes/unix_sigpipe/unix_sigpipe-struct.stderr

@@ -0,0 +1,8 @@
+error: `unix_sigpipe` attribute can only be used on `fn main()`
+  --> $DIR/unix_sigpipe-struct.rs:3:1
+   |
+LL | #[unix_sigpipe = "inherit"]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+