Properly export ET_USE_THREADPOOL from the threadpool extension (#8947)

We had a bunch of targets that would define this macro ad-hoc. It's supposed to indicate that the threadpool extension is available, so just make sure that we have it as a PUBLIC target_compile_definition in CMake and an exported_preprocessor_flags entry in Buck for extension_threadpool.

Test Plan: CI on following NOCOMMIT PR, which fails builds if ET_USE_THREADPOOL is not defined in affected places.

Needed for #8932.