Add getpayloadl to libnldbl.

This patch adds getpayloadl to libnldbl, missed in my patch that
originally implemented getpayload functions.

Tested for powerpc.

	* sysdeps/ieee754/ldbl-opt/nldbl-getpayload.c: New file.
	* sysdeps/ieee754/ldbl-opt/Makefile (libnldbl-calls): Add
	getpayload.
	(CFLAGS-nldbl-getpayload.c): New variable.