Concurrency support

[dz333]

From @gharrma on December 14, 2016 1:13

This includes:

• Methods marked with the synchronized keyword (use try-finally)
• Synchronized blocks (use try-finally)
• Fields marked with the volatile keyword
• (Probably) compile the garbage collector with new flags
• Make sure JNI environment objects are thread-local
• Some reasonable approximation of the Java memory model
• Thread-safe runtime code (especially jni.cpp, jvm.cpp, and unsafe.cpp).

Each class instance will need its own mutex and condition variable---ideally initialized lazily!

Copied from original issue: gharrma/polyllvm#5