linking fails when compiled with clang and -fsanitize=undefined

The int128 support in SimpleAtoi introduced in abseil/abseil-cpp@4b915e7 triggers this bug in Clang: [__int128_t multiply with -fsanitize=undefined fails to link (undefined reference to __muloti4)](https://bugs.llvm.org/show_bug.cgi?id=16404)



```

2020-11-05T15:34:46.1334065Z ERROR: /build/tmp/_bazel_envoybuild/b570b5ccd0454dc9af9f65ab1833764d/external/envoy/test/integration/BUILD:254:14: Linking of rule '@envoy//test/integration:hotrestart_test' failed (Exit 1): clang failed: error executing command 

2020-11-05T15:34:46.1335612Z   (cd /build/tmp/_bazel_envoybuild/b570b5ccd0454dc9af9f65ab1833764d/execroot/envoy && \

2020-11-05T15:34:46.1336305Z   exec env - \

2020-11-05T15:34:46.1336703Z     BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1 \

2020-11-05T15:34:46.1337306Z     BAZEL_LINKLIBS=-l%:libstdc++.a \

2020-11-05T15:34:46.1337873Z     BAZEL_LINKOPTS=-lm \

2020-11-05T15:34:46.1338251Z     CC=clang \

2020-11-05T15:34:46.1338572Z     CXX=clang++ \

2020-11-05T15:34:46.1338900Z     ENVOY_ASAN=1 \

2020-11-05T15:34:46.1339237Z     ENVOY_UBSAN_VPTR=1 \

2020-11-05T15:34:46.1339644Z     PATH=/usr/sbin:/usr/bin:/sbin:/bin:/opt/llvm/bin \

2020-11-05T15:34:46.1340034Z     PWD=/proc/self/cwd \

2020-11-05T15:34:46.1340759Z   /opt/llvm/bin/clang @bazel-out/k8-dbg/bin/external/envoy/test/integration/hotrestart_test-2.params)

2020-11-05T15:34:46.1341345Z Execution platform: @rbe_ubuntu_clang//config:platform

2020-11-05T15:34:46.1341798Z ld.lld: error: undefined symbol: __muloti4

2020-11-05T15:34:46.1342380Z >>> referenced by int128_have_intrinsic.inc:251 (external/com_google_absl/absl/numeric/int128_have_intrinsic.inc:251)

2020-11-05T15:34:46.1343451Z >>>               bazel-out/k8-dbg/bin/external/com_google_absl/absl/strings/_objs/strings/numbers.pic.o:(absl::operator*(absl::int128, absl::int128))

2020-11-05T15:34:46.1344374Z clang-10: error: linker command failed with exit code 1 (use -v to see invocation)

```



**Steps to reproduce the bug**



I stumble upon this issue when running Envoy's unit tests:

1. git clone https://github.com/envoyproxy/envoy.git && cd envoy

1. update Abseil to the latest master in `bazel/repository_locations.bzl`;

1. run `ci/run_envoy_docker.sh 'ci/do_ci.sh bazel.compile_time_options'`



**What version of Abseil are you using?**



The latest master.



**What operating system and version are you using**



Ubuntu 18.04.5 LTS



**What compiler and version are you using?**



clang version 10.0.0



**What build system are you using?**



bazel 3.6.0



**Additional context**



In the referenced LLVM bug it's suggested to link with compiler-rt which contains the implementation of `__muloti4`, but it's an optional LLVM component. Is it possible to include `__muloti4` in Abseil?

[
  {
    "category": "underspecification",
    "rule": "signature.missing_system_requirement",
    "source": "failure_signature",
    "excerpt": "The int128 support in SimpleAtoi introduced in abseil/abseil-cpp@4b915e7 triggers this bug in Clang: [__int128_t multiply with -fsanitize=undefined fails to link (undefined reference to __muloti4)](https://bugs.llvm.org/show_bug.cgi?id=16404)"
  }
]