Hyperloglog ARM NEON SIMD optimization

[xbasel]

Add ARM NEON optimization for HyperLogLog

• Implement two NEON optmized functions for converting between raw and
  dense representations in HyperLogLog:

  1. hllMergeDenseNEON
  2. hllDenseCompressNEON
     These functions process 16 registers in each iteration.

• Utilize existing SIMD test in hyperloglog.tcl (previously added for
  AVX2 optimization) to validate NEON implementation

Test:
valkey-benchmark -n 1000000 --dbnum 9 -p 21111 PFMERGE z hll1{t} hll2{t}

+-------------------+-----------+-----------+---------------+
|      Metric       |  Before   |   After   | Improvement % |
+-------------------+-----------+-----------+---------------+
| Throughput (k rps)|    7.42   |   76.98   |    937.47%    |
+-------------------+-----------+-----------+---------------+
| Latency (msec)    |           |           |               |
|   avg             |   6.686   |   0.595   |     91.10%    |
|   min             |   0.520   |   0.152   |     70.77%    |
|   p50             |   7.799   |   0.599   |     92.32%    |
|   p95             |   8.039   |   0.767   |     90.46%    |
|   p99             |   8.111   |   0.807   |     90.05%    |
|   max             |   9.263   |   1.463   |     84.21%    |
+-------------------+-----------+-----------+---------------+

Hardware:

CPU: Graviton 3
Architecture:           aarch64
  CPU op-mode(s):       32-bit, 64-bit
  Byte Order:           Little Endian
CPU(s):                 64
  On-line CPU(s) list:  0-63
NUMA:
  NUMA node(s):         1
  NUMA node0 CPU(s):    0-63
Memory: 256 GB

Command stats:
Before:

cmdstat_pfmerge:calls=1000002,usec=126327984,**usec_per_call=126.33**,rejected_calls=0,failed_calls=0

After:

cmdstat_pfmerge:calls=1000002,usec=8588205,**usec_per_call=8.59**,rejected_calls=0,failed_calls=0

Improved by ~14.7x.

Functional testing command:

./runtest --single unit/hyperloglog --only "PFMERGE results with simd"  --loops 10000  --fastfail

The SIMD test randomizes input and comapres scalar vs simd results.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.03%. Comparing base (aa88453) to head (4c45315).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1859      +/-   ##
============================================
- Coverage     71.09%   71.03%   -0.07%     
============================================
  Files           123      123              
  Lines         65671    65671              
============================================
- Hits          46692    46649      -43     
- Misses        18979    19022      +43     

Files with missing lines	Coverage Δ
src/hyperloglog.c	92.23% <100.00%> (ø)

... and 13 files with indirect coverage changes

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