Before, it said "global_allocator does nothing". Now it gives you
suggestions for what to do if you want to change the global allocator
(which is likely the main reason you'd be reading the comment).
This is the first (known) step towards starting to use `unix_sigpipe` in
the wild. Eventually, `rustc_driver::set_sigpipe_handler` can be removed
and all clients can use `unix_sigpipe` instead.
For now we just start using `unix_sigpipe` in once place: `rustc`
itself.
It is easy to manually verify this change. If you remove
`#[unix_sigpipe = "sig_dfl"]` and run `./x.py build` you will get an ICE
when you do `./build/x86_64-unknown-linux-gnu/stage1/bin/rustc --help |
false`. Add back `#[unix_sigpipe = "sig_dfl"]` and the ICE disappears
again.
