RFR: 8291840: Avoid JavaCalls for setting up _java_system_loader and _java_platform_loader [v3]

[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.

Calvin Cheung has updated the pull request incrementally with one additional commit since the last revision:

  add assert to the _java_platform_loader case

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9734/files
  - new: https://git.openjdk.org/jdk/pull/9734/files/042c8979..4949166a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9734&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9734&range=01-02

  Stats: 23 lines in 3 files changed: 13 ins; 0 del; 10 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