Do not limit clang generated debug info to dwarf-2 on mac.

Dwarf-2 is treated as deprecated (at best) by llvm, and does not support
a lot of C++ language features. Most notably, it does not support
namespaces and template parameters. By not specifying the dwarf version,
the compiler can decide which version to use.

Change-Id: Ic32f9101c4db0f06a8ace8f5e04af9236d01598e
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>