[foreign-memaccess+abi] RFR: 8314071: Test java/foreign/TestByteBuffer.java timed out

[Per Minborg]

This PR proposes improving performance for two `VarHandle` objects and increasing the timeout for the tests.

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Commit messages:
 - Improve performance and increase timeout

Changes: https://git.openjdk.org/panama-foreign/pull/866/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=866&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314071
  Stats: 16 lines in 1 file changed: 0 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/866.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/866/head:pull/866

PR: https://git.openjdk.org/panama-foreign/pull/866