Feature request: add support for RISCV64

[dodomorandi]

It would be very handy to have the implementation of maybesig_setjmp_asm and JUMP_BUF_SIZE for RISCV64 architecture, which it is slowly starting to be an interesting platform. I know that it is possible to use the use_c_to_interface_with_setjmp feature, but the correct value of the buffer size it is still necessary (and I do not know how to extrapolate the right value).

I realized to need this feature because I was trying to compile the pgvecto.rs extension for PostgreSQL on a RISC V SBC.

I am trying to do that, but to be honest it is a bit out of my comfort zone and I am not sure if I am able to do that in a reasonable amount of time. Any help would be incredibly appreciated, therefore thanks in advance for everything.