Support using stdin to supply input in the as tool.
Support using stdin to supply input in the as tool.

This allows getting output from other utilities using pipe.

KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,antiagainst/SPIRV-Tools,dneto0/SPIRV-Tools,greg-lunarg/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,KhronosGroup/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,greg-lunarg/SPIRV-Tools,greg-lunarg/SPIRV-Tools,dneto0/SPIRV-Tools,umar456/SPIRV-Tools,pierremoreau/SPIRV-Tools,umar456/SPIRV-Tools,LukasBanana/SPIRV-Tools,dneto0/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,KhronosGroup/SPIRV-Tools,LukasBanana/SPIRV-Tools,LukasBanana/SPIRV-Tools,antiagainst/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,umar456/SPIRV-Tools