Define LDBL_NBIT to be a mask indicating the position of the integer

Define LDBL_NBIT to be a mask indicating the position of the integer
bit in a long double.  For architectures that don't have such a bit,
LDBL_NBIT is 0.  This makes it possible to say `mantissa & ~LDBL_NBIT'
in places that previously used an #ifdef to select the right expression.
The optimizer should dispense with the extra arithmetic when LDBL_NBIT
is 0.