RFR: 8252221: Use multiple workers for Parallel GC pre-touching [v3]

[Amit Pawar]

> Please review this change.

Amit Pawar has updated the pull request incrementally with two additional commits since the last revision:

 - Fixed build issue for disable-precompiled_headers.
 - Minor changes are done as per the feedback recieved on final patch.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/180/files
  - new: https://git.openjdk.java.net/jdk/pull/180/files/232adc93..d81c3b92

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=180&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=180&range=01-02

  Stats: 230 lines in 8 files changed: 133 ins; 89 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/180.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/180/head:pull/180

PR: https://git.openjdk.java.net/jdk/pull/180