Re-enable atomic loads and stores for all RISC-V targets

This roughly reverts PR https://github.com/rust-lang/rust/pull/66548

Atomic "CAS" are still disabled for targets without the
*“A” Standard Extension for Atomic Instructions*.
However this extension only adds instructions for operations more complex
than simple loads and stores, which are always atomic when aligned.

In the [Unprivileged Spec v. 20191213](https://riscv.org/technical/specifications/)
section 2.6 *Load and Store Instructions* of
chapter 2 *RV32I Base Integer Instruction Set* (emphasis mine):

> Even when misaligned loads and stores complete successfully,
> these accesses might run extremely slowly depending on the implementation
> (e.g., when implemented via an invisible trap). Further-more, whereas
> **naturally aligned loads and stores are guaranteed to execute atomically**,
> misaligned loads and stores might not, and hence require
> additional synchronization to ensure atomicity.

Unfortunately PR https://github.com/rust-lang/rust/pull/66548 did not provide
much details on the bug that motivated it, but
https://github.com/rust-lang/rust/issues/66240 and
https://github.com/rust-lang/rust/issues/85736 appear related
and happen with targets that do have the A extension.

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