RFR: 8279018: CRC calculation in CDS should not include _version and _head_size [v2]

[Yumin Qi]

> Please review the simple change for not including _version and _header_size in crc computation. These two fields are checked separately and they should not affect crc.
> 
> tests: tier1,tier4
> 
> Thanks
> Yumin

Yumin Qi has updated the pull request incrementally with one additional commit since the last revision:

  Restore GenericCDSFileMapHeader data layout, change compute_crc instead, add code to WhiteBox to get versions instead of hard code

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6899/files
  - new: https://git.openjdk.java.net/jdk/pull/6899/files/77eb6c44..56e42768

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6899&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6899&range=00-01

  Stats: 46 lines in 7 files changed: 33 ins; 1 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6899/head:pull/6899

PR: https://git.openjdk.java.net/jdk/pull/6899