Enable 32-bit atomics on thumbv6-none-eabi.

Will cause LLVM to emit calls to the __sync routines that were added
to compiler-builtins in rust-lang/compiler-builtins#1050.

Author: thejpster

compiler/rustc_target/src/spec/targets/thumbv6_none_eabi.rs

@@ -19,10 +19,11 @@ pub(crate) fn target() -> Target {
             llvm_floatabi: Some(FloatAbi::Soft),
             asm_args: cvs!["-mthumb-interwork", "-march=armv6", "-mlittle-endian",],
             features: "+soft-float,+strict-align,+v6k".into(),
-            // atomics not available until ARMv6T2
-            atomic_cas: false,
-            max_atomic_width: Some(0),
+            // CAS atomics are implemented in LLVM on this target using __sync* functions,
+            // which were added to compiler-builtins in https://github.com/rust-lang/compiler-builtins/pull/1050
+            atomic_cas: true,
             has_thumb_interworking: true,
+            max_atomic_width: Some(32),
             ..base::arm_none::opts()
         },
     }