Fix ROOT-5477: RootCling generates rootmaps

Rootcling can now generate rootmap files (it could before but
only as genreflex).
Two options have been added, -rml and -rmf for the rootmap lib
and rootmap file names respectively.
The rootmap file name can be omitted. In this case, the name
in the rootmap file will be inferred from the one of the rootmap
lib and the rootmap file will be created where the library is
located. In this latter case, the path to the lib will be
normalised to ./ .