[libc] Build overlay math library as shared object

This was requested by people at ARM for better incremental adoption. Since it's just the math library it shouldn't need anything beyond a cmake target marked "shared".

Locate the libc overlay math-library CMake target and inspect how existing shared-library targets are declared. Build the overlay and verify that the math library is emitted as a shared object; the change is done when the CMake configuration and build produce that shared library.

Written by the indexing model from the issue text.