Support riscv32imc-unknown-none-elf

[lu-zero]

There are few uses of Arc and those are missing for the target.
Would it be acceptable to add portable-atomic and portable-atomic-util and the required knobs to support it?

[BurntSushi]

I am okay with it as long as they are only included and used for targets that specifically need them.

Another option is to just not use an Arc on those targets and instead use a Box. It might lead to higher memory usage and longer clones. This is probably not great, specifically, for the case of TimeZone which is really designed to be cheap to clone. So probably the target specific dependency is worth it.

[BurntSushi]

I don't have any experience with targets like riscv32imac-unknown-none-elf, so this is probably going to require someone other than me to contribute support here.

My main requirement here is that Jiff does not grow more dependencies in the general case. I am okay with target specific dependencies that carry their weight. A brief glance suggests that portable-atomic does, but I wouldn't want to bring that dependency in for, say, x86-64 or aarch64.

[lu-zero]

Let me prepare something, it is riscv32imc-unknown-none-elf the problematic one.