S390: Fix introduction of __wmemcmp and weak wmemcmp symbols.

The recent commit 65f7767a
has introduced __wmemcmp and the weak alias wmemcmp.
This patch also introduces those symbols if glibc is build
with CFLAGS="-march=z13" where the ifunc is omitted.

ChangeLog:

	* sysdeps/s390/wmemcmp-vx.S: Add strong alias to
	__wmemcmp and weak alias to wmemcmp.