powerpc: Fix build of wcscpy with --disable-multi-arch

Since the commit

commit 81a14439
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Tue Feb 5 17:35:12 2019 -0200

    wcsmbs: optimize wcscat

powerpc64 and powerpc64le builds fail when configured with
--disable-multi-arch and --with-cpu=power6 (or newer), due to an
undefined reference to __GI___wcscpy.  This patch fixes this on
sysdeps/powerpc/powerpc64/power6/wcscpy.c, which is only used when
multi-arch is disabled.

This patch does nothing for the failures on 32-bits powerpc builds,
because the file is under the powerpc64 subdirectory, however, powerpc
builds were already failing with --disable-multi-arch, with multiple
error messages, even before the aforementioned commit.

Tested for powerpc, powerpc64, and powerpc64le with multi-arch enabled
(all pass) and disabled (powerpc still fails as explained above).