S390: Cleanup ifunc-resolve.h.

The ifunc macros s390_vx_libc* are no longer used and
can be removed as all users are now relying on
s390_libc_ifunc_expr.

The same applies to s390_libc_ifunc.  The macro
s390_libc_ifunc_init is now renamed to
s390_libc_ifunc_expr_stfle_init and the users are
adjusted accordingly.

ChangeLog:

	* sysdeps/s390/multiarch/ifunc-resolve.h
	(s390_vx_libc_ifunc, s390_vx_libc_ifunc_redirected,
	s390_vx_libc_ifunc2, s390_vx_libc_ifunc_init,
	s390_vx_libc_ifunc2_redirected, s390_libc_ifunc):
	Delete macro definition.
	(s390_libc_ifunc_init): Rename to
	s390_libc_ifunc_expr_stfle_init.
	* sysdeps/s390/bzero: Use
	s390_libc_ifunc_expr_stfle_init instead of
	s390_libc_ifunc_init.
	* sysdeps/s390/memcmp.c: Likewise.
	* sysdeps/s390/memcpy.c: Likewise.
	* sysdeps/s390/mempcpy.c: Likewise.
	* sysdeps/s390/memset.c: Likewise.