Modify the instructions emited for PREFETCH on arm64 (#10117)

Modify the instructions emited for PREFETCH on arm64 (#10117)

Summary:
__builtin_prefetch(...., 1) prefetches into the L2 cache on x86 while the same
emits a pldl3keep instruction on arm64 which doesn't seem to be close enough.

Testing on a Graviton3, and M1 system with memtablerep_bench fillrandom and
skiplist througpuh increased as follows adjusting the 1 to 2 or 3:
```
           1 -> 2     1 -> 3
----------------------------
Graviton3   +10%        +15%
M1          +10%        +10%
```

Given that prefetching into the L1 cache seems to help, I chose that conversion

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10117

Reviewed By: pdillinger

Differential Revision: D37120475

fbshipit-source-id: db1ef43f941445019c68316500a2250acc643d5e