xnrm2 wrong on Power8

``` fortran
program test

    real*8 :: a(2), dnrm2, x

    write(*,*) 'input 0 please'
    read(*,*) x
    a(1) = 0
    a(2) = -1/x

    x = dnrm2(2_8, a, 1)

    write(*,*) x

end program
```

writes `nan` but should be `infinity`.
