Migrate libc benchmark instructions to ninja.

[MaskRay]

For documentation only change, please add [docs] or similar to the subject.

[asteinhauser]

In D82143#2102545, @MaskRay wrote:

For documentation only change, please add [docs] or similar to the subject.

OK. I'll do the next time.

[MaskRay]

Hi, your git commit contains extra Phabricator tags. You can drop Reviewers: Subscribers: Tags: and the text Summary: from the git commit with the following script:

arcfilter () {
        arc amend
        git log -1 --pretty=%B | awk '/Reviewers:|Subscribers:/{p=1} /Reviewed By:|Differential Revision:/{p=0} !p && !/^Summary:$/ {sub(/^Summary: /,"");print}' | git commit --amend --date=now -F -
}

Reviewed By: is considered important by some people. Please keep the tag. (I have updated my script to use --date=now (setting author date to committer date))

https://reviews.llvm.org/D80978 contains a git pre-push hook to automate this.

libc/benchmarks/README.md

 ### Run and display `memcpy` benchmark
 
 The following commands will run the benchmark and display a 95 percentile
 confidence interval curve of **time per copied bytes**. It also features **host
 informations** and **benchmarking configuration**.
 
 ```shell
 cd llvm-project
-cmake -B/tmp/build -Sllvm -DLLVM_ENABLE_PROJECTS='clang;clang-tools-extra;libc' -DCMAKE_BUILD_TYPE=Release
-make -C /tmp/build -j display-libc-memcpy-benchmark-small
+cmake -B/tmp/build -Sllvm -DLLVM_ENABLE_PROJECTS='clang;clang-tools-extra;libc' -DCMAKE_BUILD_TYPE=Release -G Ninja
+ninja -C /tmp/build display-libc-memcpy-benchmark-small
 ```
 
 The display target will attempt to open a window on the machine where you're
 running the benchmark. If this may not work for you then you may want `render`
 or `run` instead as detailed below.
 
 ## Benchmarking targets