toolchain: Update parameter parsing on Linux

Lack of parameters builds everything, as before.

   ./update_external_sources.sh
   Building glslang, LunarGLASS, spirv-tools

Modified parameters:

   --glslang-only becomes --glslang

Current parameters:

   -g | --glslang      # enable glslang
   -l | --LunarGLASS   # enable LunarGLASS
   -s | --spirv-tools  # enable spirv-tools

Mix and match:

   ./update_external_sources.sh -g -s
   Building glslang (-g)
   Building spirv-tools (-s)
1 file changed