Rollup merge of rust-lang#100621 - taiki-e:armv4t-atomics-32, r=cuviper

Pass +atomics-32 feature for {arm,thumb}v4t-none-eabi

Similar to rust-lang@89582e8, but for ARMv4t.
Pre-v6 ARM target does not have atomic CAS, except for Linux and Android where atomic CAS is provided by compiler-builtins. So, there is a similar issue as thumbv6m.

I have confirmed that enabling the `atomics-32` target feature fixes the problem in the project affected by this issue. (taiki-e/portable-atomic#28 (comment))

Closes rust-lang#100619

r? `@nikic`
cc `@Lokathor`

compiler/rustc_target/src/spec/armv4t_none_eabi.rs

@@ -38,7 +38,9 @@ pub fn target() -> Target {
             linker_flavor: LinkerFlavor::Ld,
             linker: Some("arm-none-eabi-ld".into()),
             asm_args: cvs!["-mthumb-interwork", "-march=armv4t", "-mlittle-endian",],
-            features: "+soft-float,+strict-align".into(),
+            // Force-enable 32-bit atomics, which allows the use of atomic load/store only.
+            // The resulting atomics are ABI incompatible with atomics backed by libatomic.
+            features: "+soft-float,+strict-align,+atomics-32".into(),
             main_needs_argc_argv: false,
             atomic_cas: false,
             has_thumb_interworking: true,

compiler/rustc_target/src/spec/thumbv4t_none_eabi.rs

@@ -47,7 +47,9 @@ pub fn target() -> Target {
             asm_args: cvs!["-mthumb-interwork", "-march=armv4t", "-mlittle-endian",],
 
             // minimum extra features, these cannot be disabled via -C
-            features: "+soft-float,+strict-align".into(),
+            // Also force-enable 32-bit atomics, which allows the use of atomic load/store only.
+            // The resulting atomics are ABI incompatible with atomics backed by libatomic.
+            features: "+soft-float,+strict-align,+atomics-32".into(),
 
             panic_strategy: PanicStrategy::Abort,
             relocation_model: RelocModel::Static,