[sanitizer_common] mark __elf_aux_vector as weak on FreeBSD

At some point FreeBSD introduced libsys as a wrapper between syscalls
and libc, and then linking sanitized programs started failing with:

  # 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.

Author: DimitryAndric

compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cpp

@@ -57,7 +57,7 @@
 // that, it was never implemented. So just define it to zero.
 #    undef MAP_NORESERVE
 #    define MAP_NORESERVE 0
-extern const Elf_Auxinfo *__elf_aux_vector;
+extern const Elf_Auxinfo *__elf_aux_vector __attribute__ ((weak));
 extern "C" int __sys_sigaction(int signum, const struct sigaction *act,
                                struct sigaction *oldact);
 #  endif