libobjc and libjava don't use the renamed libgc library anymore. It's better to
have this change in libobjc/libjava than tracking the diffs in boehm-gc.

a multilib build configured with --enable-objc-gc works and doesn't show
regressions in the objc tests.

Committed to the branches/gcj/bdwgc-20121125-merge branch.

  Matthias