perl #126396 IRIX longdouble infinity issues

perl #126396 IRIX longdouble infinity issues

In IRIX longdouble (which uses the double-double format, bigendian)
multiplying with infinity introduces garbage bytes to the second double
of the double-double.

This garbage, in turn, seems to tickle another bug in long doubles,
in comparing infinities, where these garbage bytes errorneously matter
when they should not.

Workaround: zero these garbage bytes in multiplication.

The garbage bytes seem to appear only the multiplication, as far as
t/op/infnan.t can detect.

Even though we could place the multiplication result to a temporary NV
variable (for easier infiniteness inspection) for all the platforms and
depend on optimizer doing away with the temporary, let's be conservative.