RFR: 8256450: Add gz option to jmap to write a gzipped heap dump

[Lin Zang]

This PR add "gz" option to jmap -dump command to support generate gzipped heap dump.

example:
      jmap -dump:live,gz=1,file=dump.gz <pid>

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Commit messages:
 - 8256450: Add gz option to jmap to write a gzipped heap dump

Changes: https://git.openjdk.java.net/jdk/pull/1251/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1251&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256450
  Stats: 23 lines in 2 files changed: 21 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1251.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1251/head:pull/1251

PR: https://git.openjdk.java.net/jdk/pull/1251