RFR: 8316648: jrt-fs.jar classes not reproducible between standard and bootcycle builds

[Andrew Leonard]

jrt-fs.jar classes are currently compiled with the BootJDK being used, this has a couple of issues:
- The classes are not reproducible between a standard and bootcycle build image, since the first is compiled by jdk-N-1 and the later jdk-N
- The jrt-fs classes in a standard product image are compiled by the BootJDK (typically jdk-N-1), which means from a "secure dev" perspective they are not compiled with the latest and greatest compiler with all the latest vulnerability fixes. 

The jrt-fs classes are required to be built targeting jdk-8 compatible, there is no reason they cannot be compiled with the jdk-N "interim" compiler as part of the standard build.
This then makes the jrt-fs classes reproducible regardless of which bootjdk is used.

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Commit messages:
 - 8316648: jrt-fs.jar classes not reproducible between standard and bootcycle builds

Changes: https://git.openjdk.org/jdk/pull/15888/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15888&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316648
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15888.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15888/head:pull/15888

PR: https://git.openjdk.org/jdk/pull/15888