[mlir][aarch64] Disable bf16 tests on AArch64

This patch disables 2 bf16 tests that are currently not supported on
AArch64. I've triaged these failures and opened [1] to track this. I
don't have a simple reproducer for dense_output_bf16.mlir, but it's
rather clear that both tests fail due to missing support for `bfloat`
operations in the AArch64 backend.

I'm not sure what the path forward to enable these tests on AArch64
should be. I think that there are two options:
  * AArch64 backened gains capability to legalize these nodes containing
    `bfloat` operands, or
  * MLIR (similarly to Clang) is taught not to emit such nodes in the
    first place.

[1] #58465

Differential Revision: https://reviews.llvm.org/D136273

mlir/test/Integration/Dialect/SparseTensor/CPU/dense_output_bf16.mlir

@@ -4,6 +4,8 @@
 // RUN:  -shared-libs=%mlir_lib_dir/libmlir_c_runner_utils%shlibext | \
 // RUN: FileCheck %s
 
+// UNSUPPORTED: aarch64
+
 #SparseVector = #sparse_tensor.encoding<{dimLevelType = ["compressed"]}>
 #DenseVector = #sparse_tensor.encoding<{dimLevelType = ["dense"]}>
 

mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_sum_bf16.mlir

@@ -4,6 +4,8 @@
 // RUN:  -shared-libs=%mlir_lib_dir/libmlir_c_runner_utils%shlibext | \
 // RUN: FileCheck %s
 
+// UNSUPPORTED: aarch64
+
 !Filename = !llvm.ptr<i8>
 
 #SparseMatrix = #sparse_tensor.encoding<{