cmake: modularize os/CMakeLists.txt

cmake: modularize os/CMakeLists.txt

Split monolithic os target into per-store modules to improve build
system organization and dependency management.

Previously, the "os" target compiled all sources in the os/ directory
as a single unit and linked against all dependencies collectively.

Changes:
- Break os/CMakeLists.txt into smaller, store-specific modules
- Enable per-store compile options and dependency definitions
- Make dependency relationships more explicit and granular

This modularization improves build system maintainability and makes
the codebase structure clearer for future development.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>