Simplify and clean up "update" target. Also

add an SVN-UPDATE-OPTIONS variable that controls
the update behaviour. This allows to go to a common
older revision of all svn directories:

  gmake update SVN-UPDATE-OPTIONS=-r47717

would rewind both llvm and clang to a common
revision (if clang is checked out into the llvm
tree).

llvm-svn: 47717