[foreign-memaccess] [Rev 01] RFR: 8239079: Add a benchmark for concurrent use of memory access API

[Maurizio Cimadamore]

> This patch adds a basic benchmark for a memory segment used in a concurrent setting. The benchmark computes the sum of the values in a segment - both using a sequential loop and using a parallel ForkJoin-based computation.

The pull request has been updated with 1 additional commit.

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Added commits:
 - dacd90eb: Updated benchmark to add also unsafe counterparts (as baseline).

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/20/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/20/files/ca2ff4cd..dacd90eb

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/20/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/20/webrev.00-01

  Stats: 65 lines in 1 file changed: 55 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/20/head:pull/20

PR: https://git.openjdk.java.net/panama-foreign/pull/20