cache RandomDevice file from __init__

This prevents us from seeing an invalid `IOStream` object from a saved
system image, and also ensures the files are initialized once for all
threads.

stdlib/Random/src/RNGs.jl

@@ -25,9 +25,6 @@ else # !windows
 
     function getfile(rd::RandomDevice)
         devrandom = rd.unlimited ? DEV_URANDOM : DEV_RANDOM
-        # TODO: there is a data-race, this can leak up to nthreads() copies of the file descriptors,
-        # so use a "thread-once" utility once available
-        isassigned(devrandom) || (devrandom[] = open(rd.unlimited ? "/dev/urandom" : "/dev/random"))
         devrandom[]
     end
 
@@ -411,6 +408,10 @@ for T in BitInteger_types
 end
 
 function __init__()
+    @static if !Sys.iswindows()
+        DEV_RANDOM[] = open("/dev/random")
+        DEV_URANDOM[] = open("/dev/urandom")
+    end
     seed!(GLOBAL_RNG)
 end