GCC: sorry, unimplemented: 64-bit mode not compiled in

You will need both binutils and gcc configured with:

--enable-multilib

and probably:

--enable-targets=i686-pc-linux-gnu,x86_64-pc-linux-gnu

to support multilib (the -m64 and/or -m32 options). You'll also need two versions of stuff like glibc to be able to link and run the resulting binaries.

This typically means that you're using the wrong (old) compiler.

The new compilers support both -m32 and -m64. You have to set the PATH to the new compilers (in the gcc,MinGW subdirectory of Rtools) before any old compilers in Rtools.

Try updating your compiler's binary lib path to 64bit version. Other resources like lib folders also should change accordingly.