ldbl-128: Use mathx_hidden_def inplace of hidden_def

ldbl-128: Use mathx_hidden_def inplace of hidden_def

This provides a extra macro expansion before invoking
the hidden_def macro.  This is necessary to build the
ldbl-128 files as float128 correctly.

	* sysdeps/generic/math_private.h:
	(mathx_hidden_def): New macro.
	* sysdeps/ieee754/ldbl-128/s_finitel.c: Replace hidden_def with
	the above.
	* sysdeps/ieee754/ldbl-128/s_isinfl.c: Likewise.
	* sysdeps/ieee754/ldbl-128/s_isnanl.c: Likewise.