Add lld benchmarking script.

The purpose of this script is to measure the performance effect
of an lld change in a statistically sound way, automating all the
tedious parts of doing so. It copies the test case into /tmp as well as
running the test binaries from /tmp to reduce the influence on the test
machine's storage medium on the results. It accounts for measurement
bias caused by binary layout (using the --randomize-section-padding
flag to link the test binaries) and by environment variable size
(implemented by hyperfine [1]). Runs of the base and test case are
interleaved to account for environmental factors which may influence
the result due to the passage of time. The results of running hyperfine
are collected into a results.csv file in the output directory and
may be analyzed by the user with a tool such as ministat.

Requirements: Linux host, hyperfine [2] in $PATH, run from a build
directory configured to use ninja and a recent version of lld that
supports --randomize-section-padding, /tmp is tmpfs.

[1] https://github.com/sharkdp/hyperfine/blob/3cedcc38d0c430cbf38b4364b441c43a938d2bf3/src/util/randomized_environment_offset.rs#L1
[2] https://github.com/sharkdp/hyperfine

Reviewers: rnk, MaskRay, smithp35

Reviewed By: rnk

Pull Request: https://github.com/llvm/llvm-project/pull/138367

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