RFR: 8361890: Aarch64: Removal of redundant dmb from C1 AtomicLong methods

[Samuel Chee]

The current C1 implementation of AtomicLong methods
which either adds or exchanges (such as getAndAdd)
emit one of a ldaddal and swpal respectively when using
LSE as well as an immediately proceeding dmb. Since
ldaddal/swpal have both acquire and release semantics,
this provides similar ordering guarantees to a dmb.full
so the dmb here is redundant and can be removed.

This is due to both clause 7 and clause 11 of the
definition of Barrier-ordered-before in B2.3.7 of the
DDI0487 L.a Arm Architecture Reference Manual for A-profile
architecture being satisfied by the existence of a
ldaddal/swpal which ensures such memory ordering guarantees.

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

Commit messages:
 - 8361890: AArch64: Removal of redundant dmb from C1 AtomicLong methods

Changes: https://git.openjdk.org/jdk/pull/26245/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26245&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361890
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26245/head:pull/26245

PR: https://git.openjdk.org/jdk/pull/26245