RFR: 8248238: Implementation of JEP: Windows AArch64 Support [v2]

[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 refreshed the contents of this pull request, and previous commits have been removed. The
incremental views will show differences compared to the previous content of the PR. The pull request contains six new
commits since the last revision:

 - 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

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/212/files
  - new: https://git.openjdk.java.net/jdk/pull/212/files/26e4af3a..5f9b0d35

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=212&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 2 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