make clean fails in erts

[Simon Cornish]

The Apple compiler creates DWARF debugging symbols in a bundle (ie. a
directory) named <name>.dSYM
When building from ccase/r13b04_dev (and probably any build with
debugging symbols enabled) make clean fails in erts trying to remove
child_setup.dSYM

In order for make clean to work correctly, the rm command needs -r also

git fetch git://github.com/dotsimon/otp.git darwin_build_rm_dSYMN

/Simon