Update to Zstandard 1.4.5

As usual, the full release notes are found on Github:

https://github.com/facebook/zstd/releases/tag/v1.4.5

Notable changes include:

* Improved decompress performance o

Update to Zstandard 1.4.5

As usual, the full release notes are found on Github:

https://github.com/facebook/zstd/releases/tag/v1.4.5

Notable changes include:

* Improved decompress performance on amd64 and arm (5-10%
and 15-50%, respectively).
* '--patch-from' zstd(1) CLI option, which provides something like a very fast
version of bspatch(1) with slightly worse compression. See release notes.

In this update, I dropped the 3-year old -O0 workaround for an LLVM ARM bug;
the bug was fixed in LLVM SVN in 2017, but we didn't remove this workaround
from our tree until now.

MFC after: I won't, but feel free
Relnotes: yes

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