runtime: source startupRandomData from auxv AT_RANDOM on linux/arm.

Fixes #9541.

Change-Id: I5d659ad50d7c3d1c92ed9feb86cda4c1a6e62054
Reviewed-on: https://go-review.googlesource.com/2584Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/os_linux_arm.go

@@ -51,12 +51,11 @@ func setup_auxv(argc int32, argv **byte) {
 	for i := 0; auxv[i] != _AT_NULL; i += 2 {
 		switch auxv[i] {
 		case _AT_RANDOM: // kernel provides a pointer to 16-bytes worth of random data
-			if auxv[i+1] != 0 {
+			startupRandomData = (*[16]byte)(unsafe.Pointer(uintptr(auxv[i+1])))[:]
 			// the pointer provided may not be word alined, so we must to treat it
 			// as a byte array.
-				rnd := (*[16]byte)(unsafe.Pointer(uintptr(auxv[i+1])))
-				randomNumber = uint32(rnd[0]) | uint32(rnd[1])<<8 | uint32(rnd[2])<<16 | uint32(rnd[3])<<24
-			}
+			randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
+				uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
 
 		case _AT_PLATFORM: // v5l, v6l, v7l
 			t := *(*uint8)(unsafe.Pointer(uintptr(auxv[i+1] + 1)))