Update to Zstandard 1.4.5

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