add new gen/ directory for generated sources

Allow modules to generate source into $OUT/gen, which will then
be copied into $OUT/obj and $OUT/obj_$(TARGET_2ND_ARCH) as
necessary.  This allows a single build rule invocation that includes
generated source to build for the first and second architectures.

Modules will need to change calls to local-intermediates-dir into
local-generated-sources-dir.

Change-Id: I62504bad9454b3d9fde7b84ab9f0a487a2ecf0bf