[lld] [llvm] Reapply "[RISCV] Support RISCV Atomics ABI attributes (#84597)" (PR #90266)

[Fangrui Song via llvm-commits]

================
@@ -32,8 +32,21 @@ enum AttrType : unsigned {
   PRIV_SPEC = 8,
   PRIV_SPEC_MINOR = 10,
   PRIV_SPEC_REVISION = 12,
+  ATOMIC_ABI = 14,
 };
 
+namespace RISCVAtomicAbiTag {
----------------
MaskRay wrote:

scoped enum (`enum class`) is better than namespace+unscoped enum

https://github.com/llvm/llvm-project/pull/90266