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

[Alex Bradbury via llvm-commits]

================
@@ -322,6 +331,49 @@
 #--- atomic_abi_A7.s
 .attribute atomic_abi, 3
 
+
+#      UNKNOWN_UNKNOWN: BuildAttributes {
----------------
asb wrote:

The main reason it _might_ be helpful to differentiate between the attribute being present but UNKNOWN and the attribute not being present at all, is that if the attribute is there (and emitted when merging UNKNOWN with NONE) you at least know it's been processed by tools that were aware of the attribute. Is that useful? Not sure. It's probably not hugely important either way, but thought I'd check if this was by design or not.

https://github.com/llvm/llvm-project/pull/84597