RFR: 8248238: Implementation of JEP: Windows AArch64 Support

[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

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Commit messages:
 - 8248787: G1: Workaround MSVC bug
 - 8248670: Windows: Exception handling support on AArch64
 - 8248660: AArch64: Make _clear_cache and _nop portable
 - 8248659: AArch64: Extend CPU Feature detection
 - 8248656: Add Windows AArch64 platform support code
 - 8248498: Add build system support for Windows AArch64
 - 8248681: AArch64: MSVC doesn't support __PRETTY_FUNCTION__
 - 8248663: AArch64: Avoid existing macros/keywords of MSVC
 - 8248676: AArch64: Add workaround for LITable constructor
 - 8248500: AArch64: Remove the r18 dependency on Windows AArch64 (regenerate tests)
 - ... and 3 more: https://git.openjdk.java.net/jdk/compare/68da63dc...26e4af3a

Changes: https://git.openjdk.java.net/jdk/pull/212/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248238
  Stats: 4230 lines in 69 files changed: 2406 ins; 273 del; 1551 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