In the first stage of conversion, we need to preserve the AT&T style .s assembly files for use with OS X toolchains. This change allows '--keep=s' to be used with the script to preserve these files. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Jordan Justen <jordan.l.justen@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>