RFR: 8256488: [aarch64] Use ldpq/stpq instead of ld4/st4 for small copies in StubGenerator::copy_memory

[Eugene Astigeevich]

This patch fixes 27%-48% performance regressions of small arraycopies on Graviton2 (Neoverse N1) when UseSIMDForMemoryOps is enabled. For such copies ldpq/stpq are used instead of ld4/st4.
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.

The patch passed jtreg tier1-2 and all gtest tests with linux-aarch64-server-release build and UseSIMDForMemoryOps enabled.

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Commit messages:
 - 8256488: Use ldpq/stpq instead of ld4/st4 for small copies in StubGenerator::copy_memory

Changes: https://git.openjdk.java.net/jdk/pull/1293/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1293&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256488
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1293.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1293/head:pull/1293

PR: https://git.openjdk.java.net/jdk/pull/1293