In C++ Builder XE6, released a few days ago (mid-April 2014), you can now create 64-bit packages in C++.
Old outdated answer (valid for C++ Builder XE3, 4 and 5):
64-bit compilation is a missing feature for C++ packages in XE3 and XE4.
For Delphi, you can target Win32, Win64, OSX and (I think) iOS for packages.
For C++, you can only target Win32 and OSX, even if you have the 64-bit compiler installed. This is documented for XE3, and is apparently not fixed in XE4 either.
There was no QC report for this, so I have added one. Please feel free to vote for it. My rationale for its importance is that packages are an important part of the Delphi / VCL ecosystem, especially for components. While you can use Delphi-created 64-bit packages in your 64-bit C++ project, this means you can't use components written in C++ in a 64-bit program. For us, this will require rewriting our components in Delphi, or waiting until the compiler supports it.