Comment 1 for bug 453754

True, I knew about this problem and in the past I had changed all build templates to use $LIBDIR that had "lib64" value on x86_64.
I'm still not sure how to handle /lib /lib64 on x86_64.

From what I read it seems the correct way is to always use /lib64 and make /lib a symlink of it. Could you try to add in
$(prefix)/share/xbps/shutils/{configure,build,install}_funcs.sh code to use that? you'll also need to use $LIBDIR in build templates
that hardcode /lib, something like:

if [ "$xbps_machine" = "x86_64" ]; then
    LIBDIR="/lib64"
else
   LIBDIR="/lib"
fi

LIBDIR must be set in tmpl_func.sh:set_tmpl_common_vars() much like FILESDIR and PATCHESDIR are set.

For package using build_style=gnu_configure, you'll also need to use maybe "--libdir=$LIBDIR".

I know it's a hard task, but can't really test on x86_64 right now.