wcsmbs: optimize wcpncpy

This patch rewrites wcpncpy using wcslen, wmemcpy, and wmemset.  This is
similar to the optimization done on stpncpy by 48497aba.

Checked on x86_64-linux-gnu.

        * wcsmbs/wcpncpy.c (__wcpcpy): Rewrite using wcslen, wmemcpy, and
	wmemset.