RFR: JDK-8282532: Add option to explicitly set build platform and remove support for legacy cross compilation flags

[TheShermanTanker]

On Wed, 2 Mar 2022 08:11:51 GMT, TheShermanTanker <duke at openjdk.java.net> wrote:

> Currently the only other option for manually configuring the build platform while cross compiling are devkits, which don't work on certain systems and are also more focused on differentiating the build and target compilers instead. This patch adds the ability to explicitly set the build platform through a new option, which can be especially helpful for when autodetection fails and devkits cannot be relied on, and also for simpler cross compilation cases (Like the one described in building.md)
> 
> This also removes support for the legacy cross compilation flags as well.
> 
> WIP: Translation from markdown to html

Alright, it seems I may not have selected the best approach towards the issue, marking it as a draft for the time being while I communicate with the mailing list for a better solution

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PR: https://git.openjdk.java.net/jdk/pull/7656