As the title says: How to select build configuration x86/64 in Eclipse? Is that even possible?
The build configuration is not normally part of the selection within Eclipse C++ (CDT). But rather it is based on the toolchain Eclipse uses.
Put a gcc for x86_64 on your PATH and Eclipse will use it by default.
If on the other hand you want to do Cross compilation, create a Cross GCC project and provide the specific prexix/path for your compilation.
In the Wizard:
In the project settings: