RFR: 8286399: Address possibly lossy conversions in JDK Build Tools

[Naoto Sato]

Applied required casts for the upcoming warning. Verified by cherry-picking Adam's patch.

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Commit messages:
 - 8286399: Address possibly lossy conversions in JDK Build Tools

Changes: https://git.openjdk.java.net/jdk/pull/8706/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8706&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286399
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8706.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8706/head:pull/8706

PR: https://git.openjdk.java.net/jdk/pull/8706