fix: refine feature flag conditions for Arc and Rc implementations to include target_has_atomic checks

[zouguangxian]

This PR enables building serde for the riscv32im-unknown-none-elf target.

cargo build -p serde --no-default-features --features="rc,alloc" --target riscv32im-unknown-none-elf

[zouguangxian]

target_has_atomic was an unstable feature for 1.56.0.

error[E0658]: `cfg(target_has_atomic)` is experimental and subject to change
   --> serde/src/ser/impls.rs:521:55
    |
521 |     #[cfg(any(feature = "std", all(feature = "alloc", target_has_atomic = "ptr")))]
    |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #3[29](https://github.com/serde-rs/serde/actions/runs/15034267458/job/42253004053?pr=2924#step:7:30)76 <https://github.com/rust-lang/rust/issues/32976> for more information

For more information about this error, try `rustc --explain E0658`.
error: could not compile `serde` due to previous error
Error: Process completed with exit code 101.

@dtolnay, can you give some suggestions about this feature?