resolves github issues #2 and #3

With this change, there are no errors with fakeprovide under Fedora
19.  There are two fixes here:

- Remove the `rm -rf` in `%clean` and `%install`, which is no longer
  necessary, and
- Use `setarch` before calling rpmbuild for anything other than noarch
  packages.

With these changes in place, the following works correctly:

    $ sh fakeprovide syslog
    Building binary RPM.
    -rw-rw-r--. 1 lkellogg lkellogg 2320 Dec 23 15:46 fakeprovide-syslog-20131223154653-1.fc19.noarch.rpm
    $ sh fakeprovide -a i686 syslog
    Building binary RPM.
    -rw-rw-r--. 1 lkellogg lkellogg 2404 Dec 23 15:47 fakeprovide-syslog-20131223154703-1.fc19.i686.rpm
    -rw-rw-r--. 1 lkellogg lkellogg 1768 Dec 23 15:47 fakeprovide-syslog-debuginfo-20131223154703-1.fc19.i686.rpm
    $ sh fakeprovide -a x86_64 syslog
    Building binary RPM.
    -rw-rw-r--. 1 lkellogg lkellogg 2376 Dec 23 15:47 fakeprovide-syslog-20131223154710-1.fc19.x86_64.rpm
    -rw-rw-r--. 1 lkellogg lkellogg 1736 Dec 23 15:47 fakeprovide-syslog-debuginfo-20131223154710-1.fc19.x86_64.rpm