S390: Refactor gconv_simple ifunc handling.

The ifunc handling for various __gconv_transform_* functions
which are using IFUNC on s390x are adjusted in order to omit ifunc
if the minimum architecture level already supports newer CPUs by default.
Instead those functions are just an alias to the vector variants.

Furthermore the ifunc-macro s390_libc_ifunc_expr is now used instead of
s390_vx_libc_ifunc.

ChangeLog:

	* sysdeps/s390/multiarch/gconv_simple.c (ICONV_VX_IFUNC):
	Define macro dependent on HAVE_S390_MIN_Z13_ZARCH_ASM_SUPPORT.