Auto merge of #98333 - SimonSapin:riscv-atomic, r=Amanieu

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

This roughly reverts PR #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 #66548 did not provide much details on the bug that motivated it, but #66240 and #85736 appear related and happen with targets that do have the A extension.

compiler/rustc_target/src/spec/riscv32i_unknown_none_elf.rs

@@ -11,7 +11,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
             linker: Some("rust-lld".into()),
             cpu: "generic-rv32".into(),
-            max_atomic_width: Some(0),
+            max_atomic_width: Some(32),
             atomic_cas: false,
             panic_strategy: PanicStrategy::Abort,
             relocation_model: RelocModel::Static,

compiler/rustc_target/src/spec/riscv32im_unknown_none_elf.rs

@@ -11,7 +11,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
             linker: Some("rust-lld".into()),
             cpu: "generic-rv32".into(),
-            max_atomic_width: Some(0),
+            max_atomic_width: Some(32),
             atomic_cas: false,
             features: "+m".into(),
             panic_strategy: PanicStrategy::Abort,

compiler/rustc_target/src/spec/riscv32imc_unknown_none_elf.rs

@@ -11,7 +11,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Gnu(Cc::No, Lld::Yes),
             linker: Some("rust-lld".into()),
             cpu: "generic-rv32".into(),
-            max_atomic_width: Some(0),
+            max_atomic_width: Some(32),
             atomic_cas: false,
             features: "+m,+c".into(),
             panic_strategy: PanicStrategy::Abort,