Don't gzip staging tarballs.

[maleadt]

Creating the staging tarballs takes a significant portion of the time spent compiling Julia with LLVM from source. It seems a little redundant that we're doing this in the first place -- can't we just run make install again? -- but @vtjnash has said it improves atomicity, so... But at least we shouldn't be gzipping these tarballs as they compress very badly, and just using tar is way faster.

[vtjnash]

They usually don't implement a good make uninstall, which is what we need. This sounds good, though I would be curious if we could get zstd working here now too, which is supposed to basically just make the I/O faster.

[maleadt]

On my M1 with an NVME, a 4GB LLVM debug build takes:

• 0.2s to tar -cf /dev/null
• 2.3s to tar -cf llvm.tar
• 87s (!) to tar -czf, generating 885M of output
• 6.6s to tar | zstd --fast, generating 1G of output
• 3.3s to tar | zstd --fast --threads=8
• 3.7s to tar | zstd --threads=8, generating 866MB of output

So even though zstd doesn't 'just' reduce I/O time by compressing faster than I can write my NVME, the tradeoff seems very much worth it, spending a couple of seconds to reduce storage a lot!

How do we want to depend on zstd, just add it as a required build dep, autodetect its availability, or is there a way to use it from JLLs in our Makefiles?

[oscardssmith]

since zstd and gzip produce such similar sizes, it might be worth trying lz4

[vtjnash]

We could switch BinaryBuilder over to it, then it would have to be available

[maleadt]

since zstd and gzip produce such similar sizes, it might be worth trying lz4

lz4 is both slower, does not support multithreading, and compresses my LLVM debug folder worse.

[staticfloat]

We have https://github.com/JuliaBinaryWrappers/Zstd_jll.jl available. To get it during the build process, you need to basically do the same thing that we do with p7zip:

• Add an entry in deps/Versions.mk just like we have for p7zip. This sets the source version downloaded. To set the JLL version, you need to define ZSTD_JLL_VER. If we were shipping this JLL with Julia as a stdlib, we would parse it out of the stdlib/zstd_jll/Project.toml, but since we don't have one of those, we define it in this .mk file.
• Add a deps/zstd.mk, just copy deps/p7zip.mk.