[foreign-memaccess+abi] RFR: Make `UNSAFE` variable static final

[Radoslaw Smogura]

Make `UNSAFE` static final, to avoid null checks
and traps.

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Commit messages:
 - Make `UNSAFE` variable static final

Changes: https://git.openjdk.java.net/panama-foreign/pull/436/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=436&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/436.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/436/head:pull/436

PR: https://git.openjdk.java.net/panama-foreign/pull/436