Add a generic significand implementation

Create a template for significand.

	* math/Makefile (libm-calls): Move s_significandF to...
	(gen-libm-calls): ... here.
	* math/s_significand_template.c: New file.
	* math/s_significand.c: Removed.
	* math/s_significandf.c: Removed.
	* math/s_significandl.c: Removed.
	* sysdeps/ieee754/ldbl-opt/s_significand.c: Removed.
	* sysdeps/ieee754/ldbl-opt/s_significandl.c: Removed.

Signed-off-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>