[jfx17u] RFR: 8293375: add_definitions USE_SYSTEM_MALLOC when USE_SYSTEM_MALLOC is ON

[Ao Qi]

Clean backport. Verified on (after #102):
- Linux/LoongArch64 (`USE_SYSTEM_MALLOC` is `on`, the platform triggering this problem)
- Linux/x64 (`USE_SYSTEM_MALLOC` is `off`)
- Linux/aarch64 (`USE_SYSTEM_MALLOC` is `off`)

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Commit messages:
 - 8293375: add_definitions USE_SYSTEM_MALLOC when USE_SYSTEM_MALLOC is ON

Changes: https://git.openjdk.org/jfx17u/pull/103/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx17u&pr=103&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293375
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx17u/pull/103.diff
  Fetch: git fetch https://git.openjdk.org/jfx17u pull/103/head:pull/103

PR: https://git.openjdk.org/jfx17u/pull/103