[JVMCI] fix code style and Eclipse compiler warnings

JVMCI development has shared most of the development processes used for Graal which includes keeping the code style consistent through use of Checkstyle[1] and ensuring there are no compiler warnings from the Eclipse batch compiler[2].

To preserve this practice, the Checkstyle configuration used for JVMCI should be part of the JDK code base:

java -jar checkstyle-8.8.jar -f xml -c src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.services/.checkstyle_checks.xml -o checkstyleOutput.txt $(find src/jdk.internal.vm.ci/share/classes -name '*.java' | grep -v module-info)

[1] https://checkstyle.sourceforge.io/
[2] https://help.eclipse.org/2018-09/index.jsp?topic=%2Forg.eclipse.jdt.doc.user%2Ftasks%2Ftask-using_batch_compiler.htm