MAINT: Make numpy/lib/polynomial/polyval a bit faster.

Multiplying a numpy_scalar times a numpy_array is much faster than
the other way around. This PR switches the order of multiplication
in the polyval function resulting in a speedup of about 5x for scalar
values of x.

Closes #4610.
Merge pull request #5031 from charris/speedup-old-polynomial-functions

MAINT: Make numpy/lib/polynomial/polyval a bit faster.