[foreign-memaccess+abi] RFR: Add benchmark

[Per Minborg]

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

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Commit messages:
 - Add benchmark

Changes: https://git.openjdk.org/panama-foreign/pull/761/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=761&range=00
  Stats: 143 lines in 1 file changed: 143 ins; 0 del; 0 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