Macroize function declarations in math_private.h

This patch moves the declaration of many floating-point functions from
math_private.h to math_private_calls.h and macroize the declaration to
be dependent on floating-point type.  For each of float, double, and
long double, the new header file is included once.  This reduces the
amount of repetitive boilerplate that will be required when adding
float128 versions of these functions.

Tested for powerpc64le and s390x.

	* sysdeps/generic/math_private.h: Move the declaration of many
	functions to sysdeps/generic/math_private_calls.h.
	* sysdeps/generic/math_private_calls.h: New file with the
	declarations of the functions removed from math_private.h
	macroized by floating-point type.