RFR: 8301404: Replace os::malloc with os::realloc, so we only have 1 code path

[Gerard Ziemski]

Please review this enhancement, which eliminates a duplicate and almost identical path we have for malloc() and realloc(), and collapses  them down to only one codepath, by taking advantage of fact that `malloc() == realloc(NULL)`

Tested with MACH5 tier1,2,3,4,5

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Commit messages:
 - use realloc(NULL) instead of malloc() to only have one code path

Changes: https://git.openjdk.org/jdk/pull/12621/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12621&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301404
  Stats: 154 lines in 4 files changed: 31 ins; 89 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/12621.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12621/head:pull/12621

PR: https://git.openjdk.org/jdk/pull/12621