Fix UNAME_PROCESSOR detection in Mac OS X.

Add a filter expression before to apply a platform-specific patch.

Change-Id: I9fc4cf790f16255f4e807e070dbae0e5a1f28781
Reviewed-on: https://gerrit.libreoffice.org/10227Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>