Use absl::uint128 for AES random number generator

Use absl::uint128 for AES random number generator

Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/b06e719ee985ecd63e0dffbc68499549216f817f

Replace randen’s internal 128-bit integer struct, u64x2, with
absl::uint128. This eliminates some code and improves support for
big-endian platforms.

The author works at Google. Upstream applied this patch as Piper
revision 383475671 and exported it to GitHub; the Applied-Upstream URL
above points to the exported commit.

Gbp-Pq: Name big-endian-random.diff