runtime: use poll rather than pollset for netpoll on AIX

    
    Updates golang/go#26634
    
    Reviewed-on: https://go-review.googlesource.com/126857

From-SVN: r263186

gcc/go/gofrontend/MERGE

-a2e0ad16555b2698df8e71f4c0fe02e185715bc1
+8997a3afcc746824cb70b48b32d9c86b4814807d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.