AArch64: Use ldpq/stpq instead of ld4/st4 for small copies in StubGenerator::copy_memory

Submitted by Evgeny Astigeevich (eastig@amazon.co.uk)

When UseSIMDForMemoryOps is on on Graviton2, there are 27%-48% performance regressions of arraycopy microbenchmarks for 70-80 bytes copies. Analysis shows the problem code is generated in StubGenerator::copy_memory:

    if (UseSIMDForMemoryOps) {
      __ ld4(v0, v1, v2, v3, __ T16B, Address(s, 0));
      __ ldpq(v4, v5, Address(send, -32));
      __ st4(v0, v1, v2, v3, __ T16B, Address(d, 0));
      __ stpq(v4, v5, Address(dend, -32));
    } else {

Using ldpq/stpq instead of ld4/st4 fixes the regressions. This follows what the Arm Optimization Guide, including for Neoverse N1, recommends: Use discrete, non-writeback forms of load and store instructions while interleaving them.