[mlir][bazel] Add apfloat test library (llvm#168115)

The apfloat code was added in llvm#167848, and some bazel was added
in llvm#167916 but the runtime library for test-apfloat-emulation.mlir was
missed. This patch adds the appropriate target.

Author: michaelrj-google

utils/bazel/llvm-project-overlay/mlir/BUILD.bazel

@@ -9794,6 +9794,29 @@ cc_binary(
     deps = [":mlir_float16_utils"],
 )
 
+cc_library(
+    name = "_mlir_apfloat_utils",
+    srcs = ["lib/ExecutionEngine/APFloatWrappers.cpp"],
+    defines = ["mlir_apfloat_wrappers_EXPORTS"],
+    includes = ["include"],
+    deps = [
+        "//llvm:Support",
+    ],
+)
+
+# Indirection to avoid 'libmlir_apfloat_utils.so' filename clash.
+alias(
+    name = "mlir_apfloat_utils",
+    actual = "_mlir_apfloat_utils",
+)
+
+cc_binary(
+    name = "libmlir_apfloat_utils.so",
+    linkshared = True,
+    linkstatic = False,
+    deps = [":mlir_apfloat_utils"],
+)
+
 # Unlike mlir_float16_utils, mlir_c_runner_utils, etc, we do *not* make
 # this a shared library: because on the CMake side, doing so causes issues
 # when building on Windows.  In particular, various functions take/return