Machine objects get kernel and subsystem properties

Meson has traditionally provided a system property to detect the system being run on. However this is not enough to reliably differentiate between e.g. an iOS platform from a watchOS one. Two new properties, namely kernel and subsystem have been added so these setups can be reliably detected.

These new properties are not necessary in cross files for now, but if they are not defined and a build file tries to access them, Meson will exit with a hard error. It is expected that at some point in the future defining the new properties will become mandatory.