x86_64: Fix missing wcsncat function definition without multiarch (x86-64-v4)

This code expects the WCSCAT preprocessor macro to be predefined in case
the evex implementation of the function should be defined with a name
different from __wcsncat_evex.  However, when glibc is built for
x86-64-v4 without multiarch support, sysdeps/x86_64/wcsncat.S defines
WCSNCAT variable instead of WCSCAT to build it as wcsncat.  Rename the
variable to WCSNCAT, as it is actually a better naming choice for the
variable in this case.

Reported-by: Kenton Groombridge
Link: https://bugs.gentoo.org/921945


Fixes: 64b8b651 ("x86: Add evex optimized functions for the wchar_t strcpy family")
Signed-off-by: Gabi Falk <gabifalk@gmx.com>
Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>