[lld] Fix elf::unlinkAsync detached thread
So this patch just make sure that the thread is at least stated
before we return from main.
If we just detach then the thread may be actually be stated just after
the process returned from main and it's calling atexit handers. Then the thread may try to create own function static variable and it will
add new at exit handlers confusing libc.
GLIBC before 2.27 had race in that case which corrupted atexit handlers
list. Support for this use-case for other implementation is also unclear,
so we can try just avoid that.
Reviewers: ruiu, espindola
Subscribers: emaste, arichardson, jfb, llvm-commits
Differential Revision: https://reviews.llvm.org/D58246