Use libm_alias_float128 more in sysdeps/ieee754/float128.

Use libm_alias_float128 more in sysdeps/ieee754/float128.

This patch uses libm_alias_float128 in place of weak_alias more in
sysdeps/ieee754/float128, in preparation for defining _Float64x
aliases when appropriate.

Tested for x86_64, and for powerpc64le (compilation only) with
build-many-glibcs.py in conjunction with _Float64x support patches.

	* sysdeps/ieee754/float128/s_fromfpf128.c (fromfpf128): Define
	using libm_alias_float128.
	* sysdeps/ieee754/float128/s_fromfpxf128.c (fromfpxf128):
	Likewise.
	* sysdeps/ieee754/float128/s_setpayloadf128.c (setpayloadf128):
	Likewise.
	* sysdeps/ieee754/float128/s_setpayloadsigf128.c
	(setpayloadsigf128): Likewise.
	* sysdeps/ieee754/float128/s_ufromfpf128.c (ufromfpf128):
	Likewise.
	* sysdeps/ieee754/float128/s_ufromfpxf128.c (ufromfpxf128):
	Likewise.