Intrinsic for AArch64 Arrays.fill(array, constant)

[JohnTortugo]

Please, consider this patch to intrinsify Arrays.fill(array, constant) for byte, short and int on AArch64 backend. The assembly code is a port of the HotSpot assembly for the same intrinsic. Thank you @gergo- for reviewing the draft version of this PR!

The below is a summary of some quick JMH tests that I ran on my dev machine.

Instr     Benchmark                       (size)   Mode  Cnt          Score          Error  Units
No        ArrayFillBenchmark.fill_bytes       16  thrpt    5  130,858,263.770 ±  7175243.892  ops/s
Yes       ArrayFillBenchmark.fill_bytes       16  thrpt    5  268,367,821.486 ±  2717945.345  ops/s
No        ArrayFillBenchmark.fill_bytes     1024  thrpt    5    7,284,374.553 ±   131421.427  ops/s
Yes       ArrayFillBenchmark.fill_bytes     1024  thrpt    5   94,448,799.400 ±  1096754.457  ops/s
No        ArrayFillBenchmark.fill_bytes     4096  thrpt    5    1,814,548.662 ±    33455.246  ops/s
Yes       ArrayFillBenchmark.fill_bytes     4096  thrpt    5   28,970,761.402 ±  2636159.892  ops/s

No        ArrayFillBenchmark.fill_shorts      16  thrpt    5  137,210,363.401 ±  6208453.450  ops/s
Yes       ArrayFillBenchmark.fill_shorts      16  thrpt    5  285,341,885.029 ± 11301099.208  ops/s
No        ArrayFillBenchmark.fill_shorts    1024  thrpt    5    7,178,718.150 ±   741170.216  ops/s
Yes       ArrayFillBenchmark.fill_shorts    1024  thrpt    5   53,996,849.866 ±  2353229.277  ops/s
No        ArrayFillBenchmark.fill_shorts    4096  thrpt    5    1,814,690.509 ±    13988.852  ops/s
Yes       ArrayFillBenchmark.fill_shorts    4096  thrpt    5   14,905,037.102 ±  1719260.337  ops/s

No        ArrayFillBenchmark.fill_ints        16  thrpt    5  132,083,594.942 ±  7995181.451  ops/s
Yes       ArrayFillBenchmark.fill_ints        16  thrpt    5  374,192,809.496 ±  5778630.471  ops/s
No        ArrayFillBenchmark.fill_ints      1024  thrpt    5    7,044,324.507 ±   130830.618  ops/s
Yes       ArrayFillBenchmark.fill_ints      1024  thrpt    5   28,677,308.237 ±  1515323.358  ops/s
No        ArrayFillBenchmark.fill_ints      4096  thrpt    5    1,648,798.029 ±     9113.592  ops/s
Yes       ArrayFillBenchmark.fill_ints      4096  thrpt    5    7,563,740.695 ±   627888.347  ops/s

[gergo-]

Thanks for keeping up this work! I left some high-level comments for now. I'll do some testing and a more thorough review of the assembly code soon.