fix autogenerated container_node_sizes_impl.hpp for armv7 on QNX

[asobhy-qnx]

fix #87 by checking if we are building on linux host and the target is armv7 then the use must provide a manually generated container_node_sizes_impl.hpp by providing the file under the tool directory then cmake will add it to source before building instead of autogenerating.

[foonathan]

Alright, thank you!