Fixed KRuntime to build on mono/Linux

A Lot of changes were required to the build script, but most of the changes are just addition of if='IsMono' and if='!IsMono'. Highlights of some changes, have to add Linux directory to the nuspec folder, because the the current .nuspec files have file paths that follow windows file system structure. so i rewrite them and placed them  in the nuspec\Linux folder and directed build script to them if were are on Linux/mono and use the windows versions if we are on windows. Their is only one change in the source code which was a trivial one. I am here to answer any query about the changes i have a reason for every change.Personally tested it on both windows and Linux.
I agree with @davidfowl. Having it be in lower case makes sense, it's less confusing that way. build.cmd also attempts to use the lower case name on disk - https://github.com/aspnet/KRuntime/blob/dev/build.cmd#L17