RFR: 8273152: Refactor CDS FileMapHeader loading code [v6]

[Yumin Qi]

> Please review,
>   Refactor fundamental CDS FileMapHeader code for reliable reading of basic info from shared archive.
>   With the change, it makes it possible to read an archive generated by different version of hotspot. Also it is possible to automatically generate a CDS archive If the archive supplied is not readable or fails to pass the check.
> 
>   Tests: tier1-4
>    jtreg on sa.
> 
> Thanks
> Yumin

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

  Fixed comment for magic check, fixed _file_offset to _header_size

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5768/files
  - new: https://git.openjdk.java.net/jdk/pull/5768/files/1a88076f..b804dbc6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5768&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5768&range=04-05

  Stats: 8 lines in 1 file changed: 3 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5768.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5768/head:pull/5768

PR: https://git.openjdk.java.net/jdk/pull/5768