RFR: 8291840: Avoid JavaCalls for setting up _java_system_loader and _java_platform_loader

[Calvin Cheung]

Please review this small optimization for setting up the `_java_system_loader `and `_java_platform_loader`.

I saw startup perf improvements with HelloWorld on linux-x64 like the following:


instr delta =      -102248    -0.1226%
time  delta =       -0.125 ms -0.3507%


Passed tiers 1,2,3 testing.

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Commit messages:
 - 8291840: Avoid JavaCalls for setting up _java_system_loader and _java_platform_loader

Changes: https://git.openjdk.org/jdk/pull/9734/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9734&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291840
  Stats: 41 lines in 4 files changed: 29 ins; 5 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/9734.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9734/head:pull/9734

PR: https://git.openjdk.org/jdk/pull/9734