Since you worked at Google, you know of course that the Go team is a small team working on a free software project. There are no paid QA people.
Thanks for the list of system calls. It's hard to associate that back to Go. As you know the Go library is not the C library. There is no support in Go for direct calls to functions like posix_spawn, pthread_create, pthread_mutex_lock, etc. These make no sense in the Go context. Go has its own facilities that provide similar functionality. Similarly for signal handling, process control, etc. Any programs that call the functions you list have to use C, not Go, on any platform--and Go supports calling C functions in Darwin, and those C functions will use the usual library interface.
So it's still not clear to me that there is an actual problem.