[foreign-memaccess] RFR: Add benchmark for floating point memory access var handle performance

[Maurizio Cimadamore]

This patch adds a benchmark which reproduces the issue discovered by Antoine:

https://mail.openjdk.java.net/pipermail/panama-dev/2020-September/010869.html

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Commit messages:
 - Fix benchmark to reuse outputs
 - Fix benchmark
 - Add new floating point bench

Changes: https://git.openjdk.java.net/panama-foreign/pull/386/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=386&range=00
  Stats: 131 lines in 1 file changed: 131 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/386.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/386/head:pull/386

PR: https://git.openjdk.java.net/panama-foreign/pull/386