RFR: 8248238: Implementation of JEP: Windows AArch64 Support

[Erik Joelsson]

On Wed, 16 Sep 2020 20:26:10 GMT, Monica Beckwith <mbeckwit at openjdk.org> wrote:

> This is a continuation of https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-August/009566.html
>  
> Changes since then:
> * We've improved the write barrier as suggested by Andrew [1]
> * The define-guards around R18 have been changed to `R18_RESERVED`. This will be enabled for Windows only for now but
>   will be required for the upcoming macOS+Aarch64 [2] port as well.
> * We've incorporated https://github.com/openjdk/jdk/pull/154 by @AntonKozlov in our PR for now and built the
>   Windows-specific CPU feature detection on top of it.
> 
> [1] https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-August/009597.html
> [2] https://openjdk.java.net/jeps/8251280

Build changes look good to me. I will take this branch for a spin.

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PR: https://git.openjdk.java.net/jdk/pull/212