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

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

Author: SimonSapin

compiler/rustc_target/src/spec/riscv32i_unknown_none_elf.rs

@@ -12,7 +12,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
             linker: Some("rust-lld".into()),
             cpu: "generic-rv32".into(),
-            max_atomic_width: Some(0),
+            max_atomic_width: Some(32),
             atomic_cas: false,
             executables: true,
             panic_strategy: PanicStrategy::Abort,

compiler/rustc_target/src/spec/riscv32im_unknown_none_elf.rs

@@ -12,7 +12,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
             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(),
             executables: true,

compiler/rustc_target/src/spec/riscv32imc_unknown_none_elf.rs

@@ -12,7 +12,7 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
             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(),
             executables: true,