RFR: 8254827: JVMCI: Enable it for Windows+AArch64 [v3]

[Bernhard Urban-Forster]

> Use r18 as allocatable register on Linux only.
> 
> A bootstrap works now (it has been crashing before due to r18 being allocated):
> $
> ./windows-aarch64-server-fastdebug/bin/java.exe -XX:+UnlockExperimentalVMOptions -XX:+UseJVMCICompiler -XX:+BootstrapJVMCI -version
> Bootstrapping JVMCI................................. in 17990 ms (compiled 3330 methods)
> openjdk version "16-internal" 2021-03-16
> OpenJDK Runtime Environment (fastdebug build 16-internal+0-adhoc.NORTHAMERICAbeurba.openjdk-jdk)
> OpenJDK 64-Bit Server VM (fastdebug build 16-internal+0-adhoc.NORTHAMERICAbeurba.openjdk-jdk, mixed mode)
> 
> Jtreg tests `test/hotspot/jtreg/compiler/jvmci` are passing as well.

Bernhard Urban-Forster has updated the pull request with a new target base due to a merge or a rebase. The incremental
webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits
since the last revision:

 - add missing precompiled.hpp include
 - Merge remote-tracking branch 'upstream/master' into 8254827-enable-jvmci-win-aarch64
 - rename argument to canUsePlatformRegister
 - comment for platformRegister
 - 8254827: JVMCI: Enable it for Windows+AArch64
   
   Use r18 as allocatable register on Linux only.
   
   A bootstrap works now (it has been crashing before due to r18 being allocated):
   ```console
   $
   ./windows-aarch64-server-fastdebug/bin/java.exe -XX:+UnlockExperimentalVMOptions -XX:+UseJVMCICompiler -XX:+BootstrapJVMCI -version
   Bootstrapping JVMCI................................. in 17990 ms (compiled 3330 methods)
   openjdk version "16-internal" 2021-03-16
   OpenJDK Runtime Environment (fastdebug build 16-internal+0-adhoc.NORTHAMERICAbeurba.openjdk-jdk)
   OpenJDK 64-Bit Server VM (fastdebug build 16-internal+0-adhoc.NORTHAMERICAbeurba.openjdk-jdk, mixed mode)
   ```
   
   Jtreg tests `test/hotspot/jtreg/compiler/jvmci` are passing as well.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/685/files
  - new: https://git.openjdk.java.net/jdk/pull/685/files/28dcf572..7e6cb739

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=685&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=685&range=01-02

  Stats: 29566 lines in 423 files changed: 18920 ins; 8788 del; 1858 mod
  Patch: https://git.openjdk.java.net/jdk/pull/685.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/685/head:pull/685

PR: https://git.openjdk.java.net/jdk/pull/685