RFR: 8254827: JVMCI: Enable it for Windows+AArch64

[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.

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

Commit messages:
 - 8254827: JVMCI: Enable it for Windows+AArch64

Changes: https://git.openjdk.java.net/jdk/pull/685/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=685&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254827
  Stats: 15 lines in 3 files changed: 8 ins; 0 del; 7 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