RFR: 8314114: Fix -Wconversion warnings in os code, primarily linux

[Coleen Phillimore]

On Thu, 10 Aug 2023 15:22:07 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change fixes various -Wconversion warnings from files in the os directories and related, widening types or adding checked_cast<> where narrowing.
> Tested with tier1 on linux/macosx/windows and tier1-4 on linux-x64-debug, windows-x64-debug.

Thanks for the quick feedback.

-------------

PR Review: https://git.openjdk.org/jdk/pull/15229#pullrequestreview-1572651359