[spirv][amdgpu] Set atomic size in the clang target info (#128569)

Problem identified by Joseph. The openmp device runtime uses
__scoped_atomic_load_n and similar which presently hit

```
error: large atomic operation may incur significant performance
      penalty; the access size (4 bytes) exceeds the max lock-free size (0 bytes) [-Werror,-Watomic-alignment]
```

This is because the spirv class doesn't set the corresponding field. The
base does, but only if there's a host toolchain, which there isn't.

Author: JonChesterfield

clang/lib/Basic/Targets/SPIR.h

@@ -399,6 +399,8 @@ class LLVM_LIBRARY_VISIBILITY SPIRV64AMDGCNTargetInfo final
     HasLegalHalfType = true;
     HasFloat16 = true;
     HalfArgsAndReturns = true;
+
+    MaxAtomicPromoteWidth = MaxAtomicInlineWidth = 64;
   }
 
   bool hasBFloat16Type() const override { return true; }