[foreign-memaccess+abi] RFR: Add benchmark [v2]

[Per Minborg]

> This PR proposed to add a new benchmark outlining potential performance gains with the FFM API in internal JDK classes.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Cleanup after comments

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/761/files
  - new: https://git.openjdk.org/panama-foreign/pull/761/files/bd8531ff..3b27bcf6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=761&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=761&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/761.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/761/head:pull/761

PR: https://git.openjdk.org/panama-foreign/pull/761