RFR: 8273153: Consolidate file_exists into os:file_exists

[Yumin Qi]

On Mon, 30 Aug 2021 21:43:22 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> This are several places to check if file exists using local similar function, file_exists. They should be consolidated into one version. The patch added a function os::file_exists to do so.
>> 
>> Tests: tier1
>
> LGTM.

@iklam @calvinccheung Thanks for review

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PR: https://git.openjdk.java.net/jdk/pull/5308