RFR: 8274615: Support relaxed atomic add for linux-aarch64

[Kim Barrett]

Please review this change to the linux-aarch64 port to support relaxed
atomic add operations.  Both ldxr/stxr and LSE-based implementations are
provided. The appropriate one to use selected at runtime, using the existing
infrastructure for that selection.

Testing:
mach5 tier1-3
Some hand-checked performance and functional testing that both LSE and
non-LSE implementations work, and that the relaxed operations are faster
than the conservative default.

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Commit messages:
 - relaxed atomic add

Changes: https://git.openjdk.java.net/jdk/pull/5785/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5785&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274615
  Stats: 64 lines in 4 files changed: 51 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5785.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5785/head:pull/5785

PR: https://git.openjdk.java.net/jdk/pull/5785