runtime: don't read /dev/random on Plan 9

runtime: don't read /dev/random on Plan 9

In CL 656755, the readRandom function was modified
to read an integer from /dev/random.

However, on Plan 9, /dev/random can only return
a few hundred bits a second.

The issue is that readRandom is called by randinit,
which is called at the creation of Go processes.

Consequently, it lead the Go programs to be very
slow on Plan 9.

This change reverts the change done in CL 656755
to make the readRandom function always returning 0
on Plan 9.

Change-Id: Ibe1bf7e4c8cbc82998e4f5e1331f5e29a047c4fc
Cq-Include-Trybots: luci.golang.try:gotip-plan9-arm

Reviewed-on: https://go-review.googlesource.com/c/go/+/663195
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>