wcsmbs: optimize wcpcpy

This patch rewrites wcpcpy using wcslen and wmemcpy.  This is
similar to the optimizatio done on stpcpy by f559d8cf.

Checked on x86_64-linux-gnu and string tests on a simulated
m68k-linux-gnu.

	* sysdeps/m68k/wcpcpy.c: Remove file.
	* wcsmbs/wcpcpy.c (__wcpcpy): Rewrite using wcslen and wmemcpy.