RFR: 8242440: use separate, destroyable JavaVM instances per libgraal compiler thread [v2]

[Doug Simon]

> Currently, libgraal runs in a single, permanent JavaVM instance loaded from libjvmcicompiler.so. This poses 2 problems:
> 
> 1. The memory used for libgraal is never released, even when the libgraal compiler queues are idle.
> 2. Since all libgraal HotSpot compiler threads share the same SVM heap, libgraal compilation is effectively paused when a GC occurs (SVM currently has a stop the world collector). The more compiler threads there are, the more often this happens.
> 
> This PR implements the following solution to these problems:
> 1. Create a new JavaVM instance per libgraal compiler thread (i.e. a separate SVM isolate per thread). This prevents GC in one libgraal thread from impacting another libgraal thread.
> 2. Destroy the JavaVM instances when a libgraal compiler thread becomes idle and create a new one when subsequent compilation requests arrive.
> 
> Most of the changes are in JVMCI specific files. The most significant change to shared code is the addition of the `JavaThread:: _libjvmci_runtime` field. This is required as any thread (not just a `CompilerThread`) can be associated with a specific `JVMCIRuntime` object.

Doug Simon has updated the pull request incrementally with six additional commits since the last revision:

 - avoid use of jobject for oop handles implementing foreign object references in libjvmci heap to HotSpot heap objects
 - inlined JVMCIRuntime::_jobjects
 - replace NULL with nullptr
 - added JVMCIRuntime_lock global
 - replace enum with const definition
 - rename thread parameter to current

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8262/files
  - new: https://git.openjdk.java.net/jdk/pull/8262/files/204952a0..fc9dd87f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8262&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8262&range=00-01

  Stats: 195 lines in 15 files changed: 18 ins; 5 del; 172 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8262.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8262/head:pull/8262

PR: https://git.openjdk.java.net/jdk/pull/8262