Add uint128_struct and its test

util/numeric/int128.h

Index: util/numeric/int128.h
diff --git a/util/numeric/int128.h b/util/numeric/int128.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_NUMERIC_INT128_H_
+#define CRASHPAD_UTIL_NUMERIC_INT128_H_
+
+#include <stdint.h>
+
+#include "build/build_config.h"
+
+namespace crashpad {
+
+//! \brief Stores a 128-bit quantity.
+//!
+//! This structure is organized so that 128-bit quantities are laid out in
+//! memory according to the system’s natural byte order. If a system offers a
+//! native 128-bit type, it should be possible to bit_cast<> between that type
+//! and this one.
+//!
+//! This structure is designed to have the same layout, although not the same
+//! field names, as the Windows SDK’s `M128A` type from `<winnt.h>`. It is
+//! provided here instead of in `compat` because it is useful outside of the
+//! scope of data structures defined by the Windows SDK.
+struct uint128_struct {
+#if defined(ARCH_CPU_LITTLE_ENDIAN) || DOXYGEN
+  //! \brief The low 64 bits of the 128-bit quantity.
+  uint64_t lo;
+
+  //! \brief The high 64 bits of the 128-bit quantity.
+  uint64_t hi;
+#else
+  uint64_t hi;
+  uint64_t lo;
+#endif
+};
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_NUMERIC_INT128_H_