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