RFR: 8282986: Inconsistent naming of "system" class loader

[Coleen Phillimore]

This is a somewhat trivial renaming.  Ran the CDS tests locally.

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Commit messages:
 - 8282986: Inconsistent naming of "system" class loader

Changes: https://git.openjdk.org/jdk/pull/9349/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9349&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8282986
  Stats: 19 lines in 5 files changed: 0 ins; 0 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/9349.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9349/head:pull/9349

PR: https://git.openjdk.org/jdk/pull/9349