RFR: 8255616: Disable AOT and Graal in Oracle OpenJDK

[Vladimir Kozlov]

We shipped Ahead-of-Time compilation (the jaotc tool) in JDK 9, as an experimental feature. We shipped Graal as an experimental JIT compiler in JDK 10. We haven't seen much use of these features, and the effort required to support and enhance them is significant. We therefore intend to disable these features in Oracle builds as of JDK 16. 

We'll leave the sources for these features in the repository, in case any one else is interested in building them. But we will not update or test them.

We'll continue to build and ship JVMCI as an experimental feature in Oracle builds.

Tested changes in all tiers.

I verified that with these changes I still able to build Graal in open repo and run graalunit testing: 

`open$ bash test/hotspot/jtreg/compiler/graalunit/downloadLibs.sh /mydir/graalunit_lib/`
`open$ bash configure --with-debug-level=fastdebug --with-graalunit-lib=/mydir/graalunit_lib/ --with-jtreg=/mydir/jtreg`
`open$ make jdk-image`
`open$ make test-image`
`open$ make run-test TEST=compiler/graalunit/HotspotTest.java`

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Commit messages:
 - 8255616: Disable AOT and Graal in Oracle OpenJDK

Changes: https://git.openjdk.java.net/jdk/pull/960/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=960&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255616
  Stats: 36 lines in 4 files changed: 21 ins; 11 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/960.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/960/head:pull/960

PR: https://git.openjdk.java.net/jdk/pull/960