Fix register_dtor issue
Fix register_dtor issue

Due to KLEE not supporting weak symbols,
register_dtor (std/src/sys/unix/thread_local_dtor.rs)
fails trying to invoke a symbolic function pointer
instead of detecting that it is null and not invoking it.

project-oak/rust-verification-tools,project-oak/rust-verification-tools,project-oak/rust-verification-tools,project-oak/rust-verification-tools