libgo: don't have .lo depend on .lo.dep

    
    Having each .lo depend on the corresponding .lo.dep caused too many
    rebuilds, because the .lo.dep files are rebuilt when Makefile changes.
    Instead, if the .lo.dep file changes, remove the .lo file.
    
    Reviewed-on: https://go-review.googlesource.com/25588

From-SVN: r239258

gcc/go/gofrontend/MERGE

-2c88d4871558f0451a0ad152a7052dcfaecb254f
+672db63f342c99bdc7ed46f040038440f429e600
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.