VSIPL16 — maxmg etc. of complex should return real values

The mag() and magsq() elementwise functions are defined to return real
views from complex views, as one would expect.

However, the maxmg, maxmgsq, minmg, and minmgsq reduction functions –
which ought to be simply the maximum/minimum of the mag and magsq
results, are defined to return complex values. They should likewise
return real values.

(Note that the Mentor Graphics implementation already does return real
values from these functions. The change is unlikely to break user code
in any case; if the user assigns the result to a complex variable, that
will still work when the functions return real values.)