GC options

I have been tinkering with the various GC configuration options. I am building on Raspbian Buster (ARM), although I see the same results on an X86_64 Stretch system. The only options that seem to work are Boehm-enabled and gengc and precisegc disabled. I would like to know what *should* work. I'd like to generate code targeted to an embedded platform.

Thanks,

Dave