Add dtype selective build for OSS

[lucylq]

🚀 The feature, motivation and pitch

Currently, executorch supports dtype selective build in buck (internal). We want to extend this to OSS with cmake build system.

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @larryliu0820 @manuelcandales

[digantdesai]

Thank you @lucylq !