RFR: 8292329: Enable CDS shared heap for zero builds

[Ioi Lam]

ZERO uses UseSerialGC by default. When we dump the default CDS archive during the build process, it fails to dump the shared heap (which requires G1GC).

The fix is to force -XX:+UseG1GC when dumping the default CDS archive during the build process.

Speed up:

(Before)
$ perf stat -r 40 ./images/jdk/bin/java -version
0.018080 +- 0.000388 seconds time elapsed ( +- 2.15% )

(After)
$ perf stat -r 40 ./images/jdk/bin/java -version
0.011986 +- 0.000205 seconds time elapsed ( +- 1.71% )

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Commit messages:
 - 8292329: Enable CDS shared heap for zero builds

Changes: https://git.openjdk.org/jdk/pull/9984/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9984&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292329
  Stats: 14 lines in 1 file changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9984.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9984/head:pull/9984

PR: https://git.openjdk.org/jdk/pull/9984