this fixes gcc: error: unrecognized command line option '-arch'

The '-arch' option is part of Apple's extensions to GCC, and it is uncompatible
with "vanilla" GCC from FSF. Also, we're not building "universal binaries".

Change-Id: I44e7c72bbb1dd4be5ac9cdbc4f210aaccea513b4
Reviewed-on: https://gerrit.libreoffice.org/10117Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>