RFR: 8343345: Use -jvmArgsPrepend for running microbenchmarks in RunTests.gmk

[Claes Redestad]

Update RunTests.gmk to use `-jvmArgsPrepend` to avoid overwriting built-in micro `jvmArgs` flags.

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Commit messages:
 - Use -jvmArgsPrepend for running microbenchmarks in RunTests.gmk

Changes: https://git.openjdk.org/jdk/pull/21800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343345
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21800/head:pull/21800

PR: https://git.openjdk.org/jdk/pull/21800