Removed x86 project

The managed part is always compiled as AnyCPU now, whereas the native
parts are always compiled for both x86 and x64.
Move native libraries to lib/[cpu] path
Removed x86 platform from the build script
Added linux to the dll.config file
Only rebuild native lib when modified
Fixed utility paths
Added nuget for native dependencies
Added explicit dependency to OpenTK
Pack OpenTK.Rift and dependencies
Explicitly mark and copy dependencies
Build linux/mac/windows nuget only on their respective platforms
Workaround for xbuild 2.8 missing %(RecursiveDir)
Fixed calling convention to cdecl
Fixed deployment of native dependencies
Place native libraries under build/ folder
Fixed mac and linux native targets
Fixed native nuget generation on mac