I built a version of the GNU binutils (including the assembler) on Cygwin under Windows, targetting 68k, several years ago. I don't remember it being particularly difficult. I imagine it would be equally easy hosted on BSD or Linux. Actually, I vaguely remember building it on a Raspberry Pi, too, but I don't have that with me now to check.
I wrote a little utility to convert the resulting ELF file into a boring binary file I could program into non-volatile RAMs using a board I added to a Raspberry Pi.
I also built the GNU C compiler, but I never got a working libc, which the C compiler relies on surprisingly often :-(