We called the target x86_64-win64. Straight forward FPC use is mingw(64) free as FPC has internal assemblers and linkers though.
The key is to use known good binutils from the "fpcbuild" release building repository. (directories install/bin*), and then just following regular crossbuilding procedures.
I attach my 64-bit build script, but it assumes the binw64 files have been set up in the path. Please rename it back to a .CMD extension. I changed it to .txt to suit the forum software.
The start compiler is the regular win32 3.2.0 release :
set FPCSTART=c:\fpc\3.2.0\bin\i386-win32\ppc386.exe
but the result is entirely 64-bit.
Make sure your %PATH% is free of other development tools. (mingw,cygwin, but also commercials like Delphi and Visual studio that might package non standard tools, most notably make and sh)
Just give it a try, and report back here with detail and we figure it out. It is not hard, and once you have a working setup, very repeatable.