Add proper CMake build for extension_parallel (#8938)

Previously it was copied in several places per executorch_srcs.cmake.

Needed for #8932

Test Plan: Compare cmake-out/executorch_srcs.cmake before/after for my usual testing cmake config with "all the CPU stuff" on; found that thread_parallel.cpp is now duplicated only in one place instead of multiple (it's in llama_runner, which needs a general fixup because it's duplicating several extensions).