Allow to configure archlibdir for multiarch
Since r39347, there is impossible to configure placement of rubylib.so when build is configured with "--with-multiarch". That is probably OK for Debian, but it breaks Fedora :/ The attached patch allows to configure the archlibdir, but I feel that it is suboptimal, since the "--with-rubyarchprefix" should probably be the parameter which influences placement of the arch specific libraries. Any chance that this patch is accepted or better if rubyarchprefix is respected for every arch specific library, including libruby.so. Thanks.