Before CL 62530 fastrand always returned non-zero value, and one
condition in sema.go depends on this behavior.

fastrand is used to generate random weight for treap of sudog, and
it is checked against zero to verify sudog were inserted into treap or
wait queue.

Since its precision is not very important for correctness, lets just
always set its lowest bit in this place.

Updates #22047
Updates #21806

Change-Id: Iba0b56d81054e6ef9c49ffd293fc5d92a6a31e9b
Reviewed-on: https://go-review.googlesource.com/68050Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>