[ORC] Add EPCGenericMemoryAccess: generic executor memory access via EPC calls.

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