RFR: 8276618: Pad cacheline for Thread::_rcu_counter

[Hamlin Li]

Currently, Thread::_rcu_counter is not padded by cacheline, it should be beneficail to do so.

The initial spebjbb test shows about 10.5% improvement of critical, and 0.7% improvement of max in specjbb2015, specjbb arguments:
  GROUP_COUNT=4
  TI_JVM_COUNT=1
  JAVA_OPTS_BE="-server -XX:+UseG1GC -Xms32g -Xmx32g"
  MODE_ARGS="-ikv"

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6246/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6246&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276618
  Stats: 8 lines in 3 files changed: 2 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6246/head:pull/6246

PR: https://git.openjdk.java.net/jdk/pull/6246