libsys: Implement safe aux vec initialization

[aokblast]

Summary

In llvm sanitizer, we directly refenrece the aux vector by using weak
symbol since some of the aux info does not appeared in elf_aux_info.
However, since the elf_aux_vector is lazy initialzed, it is possible
that elf_aux_vector will be NULL if the sanitizer initialize early
enough. Therefore, we provide a function to allow external user of
__elf_aux_vector to be properly initilized.

[aokblast]

Reference user: https://github.com/llvm/llvm-project/pull/186961
Raw __elf_aux_vec usage: https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp#L72

[kib]

lib/libsys/Symbol.map

7

why do you need the prefixed private symbol?

In fact, I think that it is ok but should have two underscores. And I prefer that we do not export any stable interface for this, i.e. remove the libsys_elf_aux_vec_addr from the version as well as the alias. Use __libsys_.. in the santitizer, which already knows to much about the C runtime internals anyway.

Also remove the proto from the public header.