Report the build's worker allocations when asked
OCaml's allocation report at exit (OCAMLRUNPARAM=v=0x400) counts only the main domain, and the build compiles on worker domains: a clean build of tests/tests reported 3.2M words instead of about 252M. With REWATCH_REPORT_WORKER_ALLOCATIONS set, the build also prints worker_allocated_words: <n>, for the compiler benchmarks.