[foreign-memaccess+abi] RFR: Minor Windows build fixes

[Jorn Vernee]

Some minor changes to fix build errors/warnings on Windows:

- Remove spurious `vprintf`  declaration in libStdLibTest.h (it is already included as part of stdio.h). This was causing an error due to a conflicting declaration.
- Remove `return` before free in benchmark lib. This was causing a warning that was then treated as an error.

Thanks,
Jorn

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Commit messages:
 - - Remove spurious vprintf declaration

Changes: https://git.openjdk.java.net/panama-foreign/pull/528/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=528&range=00
  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/528.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/528/head:pull/528

PR: https://git.openjdk.java.net/panama-foreign/pull/528