fix xxh64 related build

[ThomasWaldmann]

@LocutusOfBorg this is inspired by your debian patch.

https://salsa.debian.org/debian/borgbackup/-/blob/master/debian/patches/bundled-xxhash.patch

changes:

• the ".../xxh64" part of the path was missing in your patch.
• xxh64 belongs to checksums module (not crypto)
• getting it to compile (xxh64, zstd with bundled code or with system lib) was a bit of a pain, but I hope it works now

also, i gave xxh64 0.8.1 a try and it worked, so i updated the bundled code.

[codecov-commenter]

Codecov Report

Merging #5937 (7b3859e) into 1.1-maint (e41d8e8) will decrease coverage by 0.02%.
The diff coverage is n/a.

❗ Current head 7b3859e differs from pull request most recent head 1e88a85. Consider uploading reports for the commit 1e88a85 to get more accurate results

@@              Coverage Diff              @@
##           1.1-maint    #5937      +/-   ##
=============================================
- Coverage      80.00%   79.98%   -0.03%     
=============================================
  Files             27       27              
  Lines          10479    10482       +3     
  Branches        2146     2146              
=============================================
  Hits            8384     8384              
- Misses          1575     1579       +4     
+ Partials         520      519       -1     

Impacted Files	Coverage Δ
src/borg/helpers.py	87.69% <0.00%> (-0.25%)	⬇️
src/borg/cache.py	85.33% <0.00%> (-0.13%)	⬇️
src/borg/archiver.py	81.42% <0.00%> (+0.07%)	⬆️

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[ThomasWaldmann]

@LocutusOfBorg please review!

[ThomasWaldmann]

This needs some testing for the "system lib" as well as for the "use bundled code" case, see upper part of setup.py.

[LocutusOfBorg]

Test cases performed:

• 1.1.17 + this patch, no xxhash installed on the system (build OK)
• 1.1.17 + this patch, xxhash 0.8.1 installed on the system (build OK)
  thanks a lot for fixing this issue!

[ThomasWaldmann]

@LocutusOfBorg thanks for testing, will merge after test CI has completed.