Removed the host compilation of glslangValidator when cross compiling as this had the potential to fail quite easily. This also required a dependency of host toolsets for cross compiled builds which wasn't desirable.
For cross-compiled builds we now download the matching glslang binary release package (https://github.com/KhronosGroup/glslang/releases/download/) for the host platform and extract it into the correct place (SDK/bin). Only do this if a glslangValidator binary isn't already in the host bin directory meaning the step will only happen if a cross compiled build is the first build of the SDK on a particular host platform.

Removed duplicated code for downloading external projects using a common function defined in cmake/Functions.cmake.