Fix __muloti4 linker error on Clang 13

Add `-rtlib=compiler-rt` linker flags for the Clang 13 CI build.

Clang 13 generates calls to __muloti4 (a 128-bit overflow-checked multiplication intrinsic) for signed 64-bit multiplication with overflow detection. This symbol is provided by LLVM's compiler-rt but is missing from GNU's libgcc, which is the default runtime library on Ubuntu.

Related issues:

- https://github.com/llvm/llvm-project/issues/14841
- https://github.com/abseil/abseil-cpp/issues/841
- https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244169

PiperOrigin-RevId: 860529686
Change-Id: If58dc0328aa1dd87aef8ea02da555f57ad96b02c

.github/workflows/build.yml

@@ -114,6 +114,8 @@ jobs:
               -G Ninja
               -DCMAKE_C_COMPILER:STRING=clang-13
               -DCMAKE_CXX_COMPILER:STRING=clang++-13
+              -DCMAKE_EXE_LINKER_FLAGS:STRING=-rtlib=compiler-rt
+              -DCMAKE_SHARED_LINKER_FLAGS:STRING=-rtlib=compiler-rt
               -DMUJOCO_HARDEN:BOOL=ON
             tmpdir: "/tmp"
           - os: macos-15