RFR: 8332400: isspace argument should be a valid unsigned char

[Robert Toyonaga]

### Summary
This change ensures we don't get undefined behavior when calling[`isspace`](https://pubs.opengroup.org/onlinepubs/007904975/functions/isspace.html).  `isspace` accepts an `int` argument that "the application shall ensure is a character representable as an unsigned char or equal to the value of the macro EOF.". 

Previously, there was no checking of the values passed to `isspace`. I've replaced direct calls with a new wrapper `os::is_space` that performs a range check and prevents the possibility of undefined behavior from happening. For instances outside of Hotspot, I've added casts to `unsigned char`. 

**Testing**
- Added a new test in `test/hotspot/gtest/runtime/test_os.cpp` to check `os::is_space` is working correctly.
- tier1

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Commit messages:
 - Prevent undefined behaviour when calling isspace.

Changes: https://git.openjdk.org/jdk/pull/19567/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19567&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332400
  Stats: 31 lines in 8 files changed: 20 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/19567.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19567/head:pull/19567

PR: https://git.openjdk.org/jdk/pull/19567