[Distributed] make nextproc use atomic operations

Author: vchuravy

stdlib/Distributed/src/macros.jl

@@ -1,14 +1,10 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
-let nextidx = 0
+let nextidx = Threads.Atomic{Int}(0)
     global nextproc
     function nextproc()
-        p = -1
-        if p == -1
-            p = workers()[(nextidx % nworkers()) + 1]
-            nextidx += 1
-        end
-        p
+        idx = Threads.atomic_add!(nextidx, 1)
+        return workers()[(idx % nworkers()) + 1]
     end
 end