Add REUSE annotations for benchmarks in `collector/compile-benchmarks`

[pietroalbini]

The code in collector/compile-benchmarks is now a submodule of rust-lang/rust, and included in source tarballs we distribute. We thus should annotate its licensing information so that it shows up in the upcoming REUSE-powered licensing information.

Once rust-lang/rust upgrades to REUSE 4.x (which adds the new REUSE.toml), it should be possible to add a REUSE.toml in any sub-directory of collector/compile-benchmark, with a content like this:

version = 1

[[annotations]]
path = ["**"]
SPDX-FileCopyrightText = "Authorship / copyright line"
SPDX-License-Identifier = "SPDX-EXPRESSION"

[Kobzol]

Now that REUSE 4.0 is used in rustc, we can proceed with this. Would you prefer to put a REUSE.toml file into each compile-time benchmark directory, or have a top-level one in collector/compile-benchmarks with entries for all the benchmarks? I'd probably slightly prefer the latter, but no strong opinion.

[pietroalbini]

Both work, I don't have a strong opinion either.