[15] RFR(S) 8238758: [JVMCI] fix JVMCI jtreg events tests to work with GraalVM

[Vladimir Kozlov]

https://bugs.openjdk.java.net/browse/JDK-8238758
http://cr.openjdk.java.net/~kvn/8238758/webrev.00/

For JVMCI testing we sometimes use dummy compiler: -Djvmci.Compiler=EmptyCompiler
It does not work correctly when libgraal (in GraalVM) is used by default.
We have to explicitly disable libgraal use in such cases.

The fix is contributed by Doug and reviewed by GraalVM group and me for the fix in Graal repo.

Tested with hs-tier1 when JVMCI tests run.

Thanks,
Vladimir