RFR: 8254827: JVMCI: Enable it for Windows+AArch64 [v2]

[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 incrementally with two additional commits since the last revision:

 - rename argument to canUsePlatformRegister
 - comment for platformRegister

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/685/files
  - new: https://git.openjdk.java.net/jdk/pull/685/files/593dfdd6..28dcf572

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=685&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=685&range=00-01

  Stats: 18 lines in 2 files changed: 9 ins; 3 del; 6 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