RFR: 8303485: Replacing os.name for operating system customization

[Mandy Chung]

On Thu, 9 Mar 2023 17:03:05 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> The symbol has to match the build usage of OPENJDK_TARGET_OS, not the name.
>
> The one in the template file is independent to the build variables.

> I would argue that we should keep the replacement string matching the make variable its getting the value from. It makes it easier to trace the origin of the value.

Then we can define a new make variable that will also allow it be a different value than `OPENJDK_TARGET_OS` if desirable.

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PR: https://git.openjdk.org/jdk/pull/12931