RFR: 8248238: Implementation of JEP: Windows AArch64 Support [v7]

[Monica Beckwith]

> 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

Monica Beckwith has updated the pull request incrementally with two additional commits since the last revision:

 - os_windows: remove duplicated UMA handling
 - test_safefetch{32,N} works fine on win+aarch64

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/212/files
  - new: https://git.openjdk.java.net/jdk/pull/212/files/2b662010..68f61d60

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=05-06

  Stats: 22 lines in 2 files changed: 0 ins; 21 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/212.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/212/head:pull/212

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