Upgrade xxhash.h to latest dev (#11098)

Summary:
Upgrading xxhash.h to latest dev version as of 1/17/2023, which is d7197ddea81364a539051f116ca77926100fc77f This should improve performance on some ARM machines.

I allowed some of our RocksDB-specific changes to be made obsolete where it seemed appropriate, for example
* xxhash.h has its own fallthrough marker (which I hope works for us)
* As in Cyan4973/xxHash#549

Merging and resolving conflicts one way or the other was all that went into this diff. Except I had to mix the two sides around `defined(__loongarch64)`

How I did the upgrade (for future reference), so that I could use usual merge conflict resolution:
```
# New branch to help with merging
git checkout -b xxh_merge_base
# Check out RocksDB revision before last xxhash.h upgrade
git reset --hard 22161b7^
# Create a commit with the raw base version from xxHash repo (from xxHash repo)
git show 2c611a76f914828bed675f0f342d6c4199ffee1e:xxhash.h > ../rocksdb/util/xxhash.h
# In RocksDB repo
git commit -a
# Merge in the last xxhash.h upgrade
git merge 22161b7
# Resolve conflict using committed version
git show 22161b7:util/xxhash.h > util/xxhash.h
git commit -a
# Catch up to upstream
git merge upstream/main

# Create a different branch for applying raw upgrade
git checkout -b xxh_upgrade_2023
# Find the RocksDB commit we made for the raw base version from xxHash
git log main..HEAD
# Rewind to it
git reset --hard 2428b72
# Copy in latest raw version (from xxHash repo)
cat xxhash.h > ../rocksdb/util/xxhash.h
# Merge in RocksDB changes, use typical tools for conflict resolution
git merge xxh_merge_base
```

Branch https://github.com/facebook/rocksdb/tree/xxhash_merge_base can be used as a base for future xxhash merges.

Fixes #11073

Pull Request resolved: #11098

Test Plan:
existing tests (e.g. Bloom filter schema stability tests)

Also seems to include a small performance boost on my Intel dev machine, using `./db_bench --benchmarks=xxh3[-X50] 2>&1 | egrep -o 'operations;.*' | sort`

Fastest out of 50 runs, before: 15477.3 MB/s
Fastest out of 50 runs, after: 15850.7 MB/s, and 11 more runs faster than the "before" number

Slowest out of 50 runs, before: 12267.5 MB/s
Slowest out of 50 runs, after: 13897.1 MB/s

More repetitions show the distinction is repeatable

Reviewed By: hx235

Differential Revision: D42560010

Pulled By: pdillinger

fbshipit-source-id: c43ee52f1c5fe0ba3d6d6e4eebb22ded5f5492ea

HISTORY.md

@@ -24,6 +24,10 @@
 ### Public API Changes
 * Substantial changes have been made to the Cache class to support internal development goals. Direct use of Cache class members is discouraged and further breaking modifications are expected in the future. SecondaryCache has some related changes and implementations will need to be updated. (Unlike Cache, SecondaryCache is still intended to support user implementations, and disruptive changes will be avoided.) (#10975)
 
+### Performance Improvements
+* Updated xxHash source code, which should improve kXXH3 checksum speed, at least on ARM (#11098).
+* Improved CPU efficiency of DB reads, from block cache access improvements (#10975).
+
 ## 7.9.0 (11/21/2022)
 ### Performance Improvements
 * Fixed an iterator performance regression for delete range users when scanning through a consecutive sequence of range tombstones (#10877).