RFR: 8248238: Implementation of JEP: Windows AArch64 Support [v4]

[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 with a new target base due to a merge or a rebase. The pull request now
contains 13 commits:

 - 8248787: G1: Workaround MSVC bug
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248670: Windows: Exception handling support on AArch64
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248660: AArch64: Make _clear_cache and _nop portable
   Summary: __builtin___clear_cache, etc.
   Contributed-by: mbeckwit, luhenry, burban
 - 8248659: AArch64: Extend CPU Feature detection
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248656: Add Windows AArch64 platform support code
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248498: Add build system support for Windows AArch64
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248681: AArch64: MSVC doesn't support __PRETTY_FUNCTION__
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248663: AArch64: Avoid existing macros/keywords of MSVC
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - 8248676: AArch64: Add workaround for LITable constructor
   Reviewed-by: aph
   Contributed-by: mbeckwit, luhenry, burban
 - 8248500: AArch64: Remove the r18 dependency on Windows AArch64 (regenerate tests)
   Reviewed-by:
   Contributed-by: mbeckwit, luhenry, burban
 - ... and 3 more: https://git.openjdk.java.net/jdk/compare/7b860120...50ab8edf

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

Changes: https://git.openjdk.java.net/jdk/pull/212/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=03
  Stats: 2970 lines in 69 files changed: 2407 ins; 275 del; 288 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