RFR: 8274615: Support relaxed atomic add for linux-aarch64 [v2]

[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.

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into aarch64_touch_exp
 - relaxed atomic add

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5785/files
  - new: https://git.openjdk.java.net/jdk/pull/5785/files/be7d43e4..942221ce

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5785&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5785&range=00-01

  Stats: 11761 lines in 530 files changed: 6041 ins; 3729 del; 1991 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