Apple M1 (and possible ARM PCs): unpopulated cpus map

[kawahwookiee]

when RLock()'ing a drwmutex on an M1 mac, cpu() returns 0, but the map itself is empty:

func map_cpus() (cpus map[uint64]int) {
	cpus = make(map[uint64]int)
	return
}

Perhaps, a tmp solution: cpus = map[uint64]int{0: 0}