RFR: 8292494: Ensure SystemDictionary::set_platform_loader and set_system_loader are called only once

[Calvin Cheung]

The `SystemDictionary::set_platform_loader` and the `SystemDictionary::set_system_loader` functions should only be called once during VM startup, this simple fix ensures that.

Passed mach5 tiers 1,2,3 testing.

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Commit messages:
 - 8292494: Ensure SystemDictionary::set_platform_loader and set_system_loader are called only once

Changes: https://git.openjdk.org/jdk/pull/9988/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9988&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292494
  Stats: 6 lines in 1 file changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/9988.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9988/head:pull/9988

PR: https://git.openjdk.org/jdk/pull/9988