AArch64: Use better block copy8

AArch64: Use better block copy8

The vector copy is only necessary for 16-byte blocks on AArch64.

Decompression uplifts on a Neoverse V2 system, using Zstd-1.5.8
compiled with "-O3 -march=armv8.2-a+sve2":

                 Clang-19  Clang-20    GCC-14    GCC-15
 1#silesia.tar:   +0.316%   +0.865%   +0.025%   +0.096%
 2#silesia.tar:   +0.689%   +1.374%   +0.027%   +0.065%
 3#silesia.tar:   +0.811%   +1.654%   +0.034%   +0.033%
 4#silesia.tar:   +0.912%   +1.755%   +0.027%   +0.042%
 5#silesia.tar:   +0.995%   +1.826%   +0.062%   +0.094%
 6#silesia.tar:   +0.976%   +1.777%   +0.065%   +0.104%
 7#silesia.tar:   +0.910%   +1.738%   +0.077%   +0.110%