support using 'system' as alias for 'dummy' toolchain

[boegel]

This enables using 'system' toolchain in easyconfigs, either directly:

toolchain = {'name': 'system', 'version': 'system'}

or via the SYSTEM constant:

toolchain = SYSTEM

This is done in preparation for EasyBuild 4.x, where the dummy toolchain is deprecated and replaced by the new system toolchain with slightly different behavior w.r.t. toolchain version and loading of dependencies in the build environment (see #2877).

The alias approach used here allows us to avoid the extensive work done in #2877 to properly handle allowing both using system and dummy as toolchain name.