RFR: 8262011: [JVMCI] allow printing to tty from unattached libgraal thread

[Doug Simon]

Currently, `HotSpotJVMCIRuntime.writeDebugOutput` does nothing if the current thread is not attached to HotSpot (i.e., `Thread::current_or_null() == NULL`). This means crucial debug info can be lost. For reference, an unattached libgraal thread is a thread started from within libgraal that has not yet attached itself to the VM (e.g., before [this line](https://github.com/oracle/graal/blob/e4b9ab931940e1946f96f2015b937ba100384573/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java#L42)) or has already detached itself (e.g., after [this line](https://github.com/oracle/graal/blob/e4b9ab931940e1946f96f2015b937ba100384573/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java#L46)).

The reason for the current behavior is that `HotSpotJVMCIRuntime.writeDebugOutput` passes a Java byte array to C++ code and the C++ code calls back into Java to decode the byte array into a native buffer. These call backs require the current thread to be attached to the VM.

This PR moves the Java-to-native-buffer decoding into Java and thus avoids the requirement for the current thread to be attached to the VM.

Tested in libgraal by patching Graal as follows:
diff --git a/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java b/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java
index 36064767c95..352395dd59b 100644
--- a/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java
+++ b/compiler/src/org.graalvm.compiler.core/src/org/graalvm/compiler/core/GraalServiceThread.java
@@ -43,7 +43,14 @@ public class GraalServiceThread extends Thread {
         try {
             runnable.run();
         } finally {
+            String debug = System.getenv("GraalServiceThread.debug");
             afterRun();
+            if ("true".equals(debug)) {
+                throw new InternalError("THROWN AFTER DETACHING");
+            }
         }
     }

Running without the changes in this PR:
> env GraalServiceThread.debug=true java -jar dacapo.jar avrora
===== DaCapo 9.12 avrora starting =====
===== DaCapo 9.12 avrora PASSED in 4270 msec =====

Running with the changes in this PR:
> env GraalServiceThread.debug=true java -jar dacapo.jar avrora
===== DaCapo 9.12 avrora starting =====
Exception in thread "LibGraalHotSpotGraalManagement-init" java.lang.InternalError: THROWN AFTER DETACHING
	at org.graalvm.compiler.core.GraalServiceThread.run(GraalServiceThread.java:52)
	at com.oracle.svm.core.thread.JavaThreads.threadStartRoutine(JavaThreads.java:519)
	at com.oracle.svm.core.posix.thread.PosixJavaThreads.pthreadStartRoutine(PosixJavaThreads.java:192)
===== DaCapo 9.12 avrora PASSED in 4688 msec =====

-------------

Commit messages:
 - 8262011: [JVMCI] allow printing to tty from unattached libgraal thread

Changes: https://git.openjdk.java.net/jdk/pull/2640/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2640&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262011
  Stats: 322 lines in 8 files changed: 49 ins; 246 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2640.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2640/head:pull/2640

PR: https://git.openjdk.java.net/jdk/pull/2640