powerpc64le: Enable float128

powerpc64le: Enable float128

This patch adds ULPs for the float128 type, updates the abilist for libc
and libm, and adds the files bits/floatn.h and float128-abi.h, in order to
enable the new type for powerpc64le.

This patch also adds the implementation of sqrtf128 for powerpc64le, since
it is not implemented in libgcc.  The sfp-machine.h header is taken from
libgcc.

Tested for powerpc64le (GCC 6.2 and GCC 7.1), powerpc64 and s390x.

	* manual/math.texi (Mathematics): Mention the enabling of float128
	for powerpc64le.
	* sysdeps/powerpc/bits/floatn.h: New file.
	* sysdeps/powerpc/fpu/libm-test-ulps: Regenerated.
	* sysdeps/powerpc/fpu/math_private.h:
	(__ieee754_sqrtf128): New inline override.
	* sysdeps/powerpc/powerpc64le/Implies-before: New file.
	* sysdeps/powerpc/powerpc64le/Makefile: New file.
	* sysdeps/powerpc/powerpc64le/fpu/e_sqrtf128.c: New file.
	* sysdeps/powerpc/powerpc64le/fpu/sfp-machine.h: New file.
	* sysdeps/powerpc/powerpc64le/power9/fpu/e_sqrtf128.c: New file.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libc-le.abilist:
	Updated.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/libm-le.abilist:
	Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64le/float128-abi.h:
	New file.