RFR: JDK-8283674: Pad ObjectMonitor allocation size to cache line size

[Thomas Stuefe]

See discussion under [1].

Since the libc malloc allocator may place ObjectMonitor instances adjacent to each other, we should pad the size of ObjectMonitor to fill a whole cache line to prevent false sharing between adjacent OMs.

[1] https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2022-March/054187.html

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Commit messages:
 - JDK-8283674-pad-om-to-cacheline-size

Changes: https://git.openjdk.java.net/jdk/pull/7955/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7955&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283674
  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7955.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7955/head:pull/7955

PR: https://git.openjdk.java.net/jdk/pull/7955