Move generic nan-pseudo-number.h to ldbl-96

The concept of pseudo number formats only exists in the realm of the
96 bit long double format.

diff --git a/sysdeps/generic/nan-pseudo-number.h b/sysdeps/ieee754/ldbl-96/nan-pseudo-number.h
similarity index 100%
rename from sysdeps/generic/nan-pseudo-number.h
rename to sysdeps/ieee754/ldbl-96/nan-pseudo-number.h