toolchain.eclass: EAPI 7 aware for D,ED,ROOT,EROOT

toolchain.eclass: EAPI 7 aware for D,ED,ROOT,EROOT

These variables may or may not have the trailing slash. Additionally,
avoid leading double slash (a network location for Cygwin) with ROOT and
EROOT because they may be empty, while D and ED never should be empty
and there is no reason so far to avoid double slashes in between.
On the other hand, eclass variables like DATAPATH and LIBPATH do contain
the leading slash, so an extra slash reduces readability for no reason.