Skip default_library = None for ToolXilinx (syn) (now work is derived
from ToolMakefile.

Prior "set_property LIBRARY {library} [get_files {srcfile}]" was set
unless library = None. Now it is set unless library is None or work.
This causes one Xilinx vivado_sim testcase to fail/change since
set_property LIBRARY work was set - removed.