RFR: 8345726: Update mx in RunTestPrebuiltSpec to reflect change in JDK-8345302

[Erik Joelsson]

In [JDK-8345302](https://bugs.openjdk.org/browse/JDK-8345302) I updated the default JAVA_FLAGS_BIG to -Xmx2048M in configure. In RunTestPrebuiltSpec.gmk, there is a copy of this value that also needs to be updated as it should mimic a typical configuration.

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Commit messages:
 - JDK-8345726

Changes: https://git.openjdk.org/jdk/pull/22615/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22615&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345726
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22615/head:pull/22615

PR: https://git.openjdk.org/jdk/pull/22615