[ORC] Add EPCGenericJITLinkMemoryManager: memory management via EPC calls.

All ExecutorProcessControl subclasses must provide a JITLinkMemoryManager object
that can be used to allocate memory in the executor process. The
EPCGenericJITLinkMemoryManager class provides an off-the-shelf
JITLinkMemoryManager implementation for JITs that do not need (or cannot
provide) a specialized JITLinkMemoryManager implementation. This simplifies the
process of creating new ExecutorProcessControl implementations.