Can't compile for arch=arm64 on OSX Yosemite terminal

In terminal prompt on OSX Yosemite I am running a command line build on some 3rd party code using xcode command line tools, I don't seem to be able to specify arch=arm64 as a flag which is recognised by the compiler

When I type c++ -v I get this

gcc version 4.2.1 (Apple Inc. build 5666) (dot 3) (MacPorts apple-gcc42 5666.3_13)

When I type cc -v

Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn) Target: x86_64-apple-darwin14.0.0 Thread model: posix

So this seems to be the problem c++ is symlinked to gcc, how do I set to clang

I have Xcode6 command line tools installed but also have a parallel copy of Xcode 5 installed so I think somewhere things have got mixed up. I need to keep both versions

Thanks