Modify files performing pluralisation checks to incorporate the
dedicated macro located in `syntax::errors`.

Adress issue #64238. Create a macro to be used for pluralisation check
throughout rustc codebase.

This means that callers can pass in a closure like
`|buf| some_function(..., &mut buf)` and pass in arbitrary arguments to
that function without complicating the trait definition. We also keep
the impl for str and String, since it's useful to be able to just pass
in "" or format!("{}"...) results in some cases.

This changes Print's definition to take self, instead of &self, because
otherwise FnOnce cannot be called directly. We could instead take FnMut
or even Fn, but that seems like it'd merely complicate matters -- most
of the time, the FnOnce does not constrain us at all anyway. If it does,
a custom Print impl for &'_ SomeStruct is not all that painful.

This avoids needlessly creating and threading the buffers through when
we only use them once.

Use it for feature-gating `#[bench]`

This fixes a case where an `ignore-musl` test was not ignored on
`armv7-unknown-linux-musleabihf` because the environment did not exactly
match. Only enforce that the environment starts with the argument to
`ignore-`.