The change in r90189 adds a link in a directory outside the iPhone platform

The change in r90189 adds a link in a directory outside the iPhone platform
directory when building the llvmCore_Embedded project.  Fix this by putting
the iPhone platform directory into DEST_DIR instead of DEST_ROOT.  I also
noticed what appears to be an unintentional use of DEVELOPER_BIN instead of
DEVELOPER_DIR, so I fixed that and changed to use DEVELOPER_DIR in some places
that were hardcoded to "Developer".  Finally, the other changes here allowed
some refactoring and simplification, which I have done.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93878 91177308-0d34-0410-b5e6-96231b3b80d8