Clarify licensing situation of MPSC and SPSC queue
Originally, these two files were licensed under the `BSD-2-Clause` license, as they were based off sample code on a blog licensing those snippets under that license:
* `library/std/src/sync/mpsc/mpsc_queue.rs`
* `library/std/src/sync/mpsc/spsc_queue.rs`
In 2017 though, the author of that blog agreed to relicense their code under the standard `MIT OR Apache-2.0` license in https://github.com/rust-lang/rust/pull/42149. This PR clarifies the situation in the files by expanding the comment at the top of the file.
r? ``@pnkfelix``
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