RFR: 8276054: JMH benchmarks for Fences

[Aleksey Shipilev]

While working on JDK-8252990, I realized there are no microbenchmarks for Fences. I would be good to add some. While technically we can benchmark `Unsafe` directly, I instead chose to use the public API, `VarHandles`, as this captures the real-world uses too.

Sample output for `make test TEST=micro:vm.fences` on `x86_64`:


Benchmark                      Mode  Cnt  Score    Error  Units
Multiple.acquire               avgt    9  0.408 ±  0.005  ns/op
Multiple.full                  avgt    9  4.694 ±  0.003  ns/op
Multiple.plain                 avgt    9  0.407 ±  0.004  ns/op
Multiple.release               avgt    9  0.407 ±  0.003  ns/op
Multiple.storeStore            avgt    9  0.409 ±  0.006  ns/op
MultipleWithLoads.acquire      avgt    9  4.600 ±  0.002  ns/op
MultipleWithLoads.full         avgt    9  8.322 ±  0.003  ns/op
MultipleWithLoads.plain        avgt    9  4.328 ±  0.001  ns/op
MultipleWithLoads.release      avgt    9  4.600 ±  0.002  ns/op
MultipleWithLoads.storeStore   avgt    9  4.600 ±  0.002  ns/op
MultipleWithStores.acquire     avgt    9  0.812 ±  0.001  ns/op
MultipleWithStores.full        avgt    9  5.291 ±  0.001  ns/op
MultipleWithStores.plain       avgt    9  0.812 ±  0.001  ns/op
MultipleWithStores.release     avgt    9  0.812 ±  0.001  ns/op
MultipleWithStores.storeStore  avgt    9  0.812 ±  0.001  ns/op
SafePublishing.plain           avgt    9  6.378 ±  0.011  ns/op
SafePublishing.release         avgt    9  6.368 ±  0.010  ns/op
SafePublishing.storeStore      avgt    9  6.376 ±  0.013  ns/op
Single.acquireFence            avgt    9  0.410 ±  0.009  ns/op
Single.fullFence               avgt    9  4.689 ±  0.002  ns/op
Single.plain                   avgt    9  0.409 ±  0.010  ns/op
Single.releaseFence            avgt    9  0.410 ±  0.006  ns/op
Single.storeStoreFence         avgt    9  0.410 ±  0.009  ns/op

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

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/6138/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6138&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276054
  Stats: 415 lines in 5 files changed: 415 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6138.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6138/head:pull/6138

PR: https://git.openjdk.java.net/jdk/pull/6138