RFR: Fix Minimal VM build failures

[Chris Plummer]

On Mon, 27 Sep 2021 18:47:33 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Minimal VM configs have no C2 and no JVMTI. New Loom code fails the Minimal VM builds because of this. See for example GHA runs: https://github.com/shipilev/loom/runs/3717293969?check_suite_focus=true
>> 
>> C2 needs just a little bit of protection when reaching to `DerivedPointersTable`.
>> 
>> JVMTI needs a bit more work. `jvmtiExport` and `jvmtiThreadState` headers are usually included even with JVMTI turned off, but `.cpp` would not be compiled without JVMTI. Therefore, non-trivial implementations should go into `.cpp`. Plus, some of the paths that call the actual methods declared but not implemented without JVMTI should be protected with `INCLUDE_JVMTI`.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server, `tier1_loom` still passes (includes JVMTI tests)
>>  - [x] Linux x86_64 minimal now builds, runs `tier1_loom` (cannot pass JVMTI tests without JVMTI)
>
> src/hotspot/share/prims/jvm.cpp line 3919:
> 
>> 3917:     JvmtiExport::post_vthread_mount(vthread);
>> 3918:   }
>> 3919: #endif
> 
> It would be bug if these functions were called when JVMTI isn't compiled in so maybe we should put an #else ShouldNotReachHere() to catch it.

I think that makes sense. At first I was thinking why not just #ifdef out the whole API, but these APIs are mapped natives for `VirtualThread.notifyJvmtiMountBegin()` and `VirtualThread.notifyJvmtiMountEnd()`. Calls to them come from java, but should only happen if jvmti is enabled.

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PR: https://git.openjdk.java.net/loom/pull/67