I’m porting the GCC and Binutils to a unique (really simple) 8-bit RISC soft processor.

I would like to be able to execute 16 and 32 bit arithmetics. The current ISA is probably not sufficient yet for a C compiler. Is there a set of requirements somewhere about the basic needs of the compiler in terms of the instructions that a new target should have? I suppose it probably needs push, pop, arithmetics with carry, shift, mov, logical etc and my question is what kind of instructions should I have of these?

I went through the documentation of the GNU GCC but I couldn’t find an exact explanation of what are the basic requirements towards a new target ISA.