Modifies the instrumented clinit_wrapper function

The previous version of clinit_wrapper did not conform to the JVM
specification (section 5.5) as it did not support concurrency. More
specifically, the clinit_wrapper has to be carefully synchronised
as other threads may try to initialize a given class or interface
at the same-time.

This commit makes the clinit_wrapper thread-safe by introducing a
critical section and two state variables. The commit implements a
simplification of the algorithm defined in section 5.5 of the JVM
specification. For instance, exceptions thrown during the execution
of static initializers are not properly handled.

Author: Degiorgio