BaseTools ConvertMasmToNasm: Support preserving assembly files

BaseTools ConvertMasmToNasm: Support preserving assembly files

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>