RFR: Use os::malloc instead of malloc

[William Kemper]

This fixes a compiler error with gcc-10.3.0 used by github actions

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Commit messages:
 - Use os::malloc instead of malloc

Changes: https://git.openjdk.org/shenandoah/pull/155/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=155&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/155.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/155/head:pull/155

PR: https://git.openjdk.org/shenandoah/pull/155