RFR: 8277878: Fix compiler tests after JDK-8275908

[Volker Simonis]

This is a quick fix for the two compiler tests introduced by JDK-8275908. The test explicitly added SerialGC in the parameter list which leads to a garbage collector conflict, if the tests are run with some other Garbage collector.

It was suggested to fix this by adding a `@requires vm.gc.Serial` tag but this is not necessary because the tests are actually GC-agnostic so I've removed the `-XX:+UseSerialGC` parameter from the test command line instead.

After the fix it was necessary to refine the check for whether the test JVM has JVMCI support built-in. Before the fix, I used  `WhiteBox.getWhiteBox().isJVMCISupportedByGC()` which worked fine if only running with SerialGC. Now that we can run with GCs which don't support JVMCI we have to use the more specific `(WB.getBooleanVMFlag("EnableJVMCI") != null)`.

Please let me know if you want me to push this instantly after it has been reviewed or if you first want to re-run your internal Tier3 tests before pushing.

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Commit messages:
 - 8277878: Fix compiler tests after JDK-8275908

Changes: https://git.openjdk.java.net/jdk/pull/6581/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6581&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277878
  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6581.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6581/head:pull/6581

PR: https://git.openjdk.java.net/jdk/pull/6581