binutils and cctools: Prefix derivation names with target for cross

We want platform triple prefixes and suffixes on derivation names to
be used consistently. The ideom this commit strives for is

 - suffix means build != host, i.e. cross *built* packages. This is
   already done.

 - prefix means build != target, i.e. cross tools. This matches the
   tradition of such binaries themselves being prefixed to disambiguate.]
   Binutils and cctools, as build tools, now use the latter