RFR: JDK-8263884: Clean up os::is_allocatable() across Posix platforms [v2]

[Thomas Stuefe]

On Wed, 24 Mar 2021 07:51:03 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   remove unnecessary cast
>
> Looks good

Thanks, @stefank and @mgkwill !

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PR: https://git.openjdk.java.net/jdk/pull/3092