Make adaptabuild a submodule

[rhempel]

Currently the CI pipeline has to build an environment that allows adaptabuild to create the umm_malloc library and test it.
This is a potential source of problems because we need to keep umm_malloc in sync with changes to adaptabuild - or maintain backwards compatibility in adaptabuild.
The adaptabuild system isn't very big (~100K) so we can avoid problems by simply including it as a submodule of umm-malloc - then we are guaranteed to be able to build and test umm_malloc even if adaptabuild advances.
The downside is that larger projects that use adaptabuild and have umm_malloc as a dependency could have multiple (possibly conflicting) copies of adaptabuild. For any top level project, we specify one (and only one) adaptabuild folder. This is usually in the root of the project. Even if there are additional adaptabuild folders further down in the tree, they are invisible to the top layer adpatabuild.