RFR: 8301971: Make JDK source code UTF-8 [v2]

[Magnus Ihse Bursie]

On Sun, 13 Apr 2025 23:14:41 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> Most of the JDK code base has been transitioned to UTF-8, but not all. This has recently become an acute problem, since our mixing of iso-8859-1 and utf-8 in properties files confused the version of `sed` that is shipped with the new macOS 15.4.
>> 
>> The fix is basically simple, and includes the following steps:
>> * Look through the code base for text files containing non-ASCII characters, and convert them to UTF-8, if they are not already
>> * Update tooling used in building to recognize the fact that files are now in UTF-8 and treat them accordingly (basically, updating compiler flags, git attributes, etc).
>
> Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Also tell javadoc that we have utf-8 now

Inspired by [Phil's comment in JDK-8353948](https://bugs.openjdk.org/browse/JDK-8353948?focusedId=14769043&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14769043), I also modified configure to only allow utf-8 environments, but to also allow `en_US.UTF-8` as a valid locale. 

This also resolves [JDK-8333247](https://bugs.openjdk.org/browse/JDK-8333247) in a better way.

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PR Comment: https://git.openjdk.org/jdk/pull/24574#issuecomment-2800741990