Support platforms that do not have native atomics

[lu-zero]

Fixes #162

e.g. for esp32c3 and such you can see it building using

cargo build --target=riscv32imc-unknown-none-elf --features alloc --no-default-features --features portable-atomic/unsafe-assume-single-core

[BurntSushi]

I've fixed this PR up a little bit:

• The mod sync was declared pub(crate) unnecessarily.
• I added some light docs.
• I added a CI job to test this.
• Most critically, I stopped enabling alloc unconditionally in portable-atomic-util. Jiff requires alloc opt-in, so I moved it to alloc = ["serde?/alloc", "portable-atomic-util/alloc"]. (I think this doesn't matter so much right now since Jiff doesn't compile in no-alloc mode, but I hope it will soon and this would have been easy to overlook!)