At some point FreeBSD introduced libsys as a wrapper between syscalls
and libc, and then linking sanitized programs started failing with:
```text
# c++ -fsanitize=address main.cc
ld: error: undefined symbol: __elf_aux_vector
>>> referenced by sanitizer_linux_libcdep.cpp:950 (/usr/src/contrib/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp:950)
>>> sanitizer_linux_libcdep.o:(__sanitizer::ReExec()) in archive /usr/lib/clang/17/lib/freebsd/libclang_rt.asan-x86_64.a
c++: error: linker command failed with exit code 1 (use -v to see invocation)
```
Mark `__elf_aux_vector` as weak in the internal sanitizer declaration,
so the linker will accept it at link time. The dynamic linker will then
take care of the symbol at run time.
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