RFR: 8273152: Refactor CDS FileMapHeader loading code [v8]

[Yumin Qi]

On Thu, 7 Oct 2021 06:06:02 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Yumin Qi has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add offset check for _generic_header in populate_header
>
> LGTM

@iklam @calvinccheung Thanks for review!

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