Prefer NV instead of long double.

Prefer NV instead of long double.

Background: sprintf aka sv_vcatpvn_flags uses a long double for
floating point values whenever has-long-double, not only when
use-long-double.  The (only?) reason for this is being able to
(perlio) printf long doubles from XS, even if no use-long-double.
(see ext/XS-APItest/t/printf.t)

Instead of casting the long double (explicitly or implicitly),
try keeping also an NV in sync, and using it.  Could probably
use the NV even more, but trying to stay minimal given the
impending 5.22.