Per discussion on gn-dev:

While trying to write templates to generate mac framework bundle, we were investigating whether we could put the generated dynamic library in a sub-directory of root_build_dir. Mostly in a (maybe misdirected) attempt to keep build_root_dir clean. As we have to copy the dynamic library to the bundle, we can change the name at that point, so having the default name is not a problem.

I'll see if it is possible to write a simple CL to add a "output_dir" parameter to executable/dynamic_library/static_library targets. Then it'll be clearer whether this is worth supporting or not.