RFR: 8314114: Fix -Wconversion warnings in os code, primarily linux [v3]

[Coleen Phillimore]

> 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.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  David's suggested changes.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15229/files
  - new: https://git.openjdk.org/jdk/pull/15229/files/fd5464d7..6777a2a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15229&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15229&range=01-02

  Stats: 17 lines in 4 files changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/15229.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15229/head:pull/15229

PR: https://git.openjdk.org/jdk/pull/15229