Refactor JITCompiledModule support for platform specialization

Add lazy CUDA symbol initialization hooks
Merge branch 'master' of github.com:halide/Halide

Conflicts:
	cpp/src/CodeGen.cpp
	cpp/src/CodeGen.h
	cpp/src/CodeGen_Internal.cpp
	cpp/src/CodeGen_Internal.h
	cpp/src/CodeGen_PTX_Host.cpp
	cpp/src/CodeGen_PTX_Host.h