llvm-shlib: Remove the option to override __cxa_atexit

If anybody is actually using this, it probably doesn't do what they
think it does. This actually causes the dylib to *export* a
__cxa_atexit symbol, so anything that links it probably loses their
exit time destructors as well as disabling LLVM's.

This just removes the option entirely. If somebody does need this
behaviour we should figure out a more principled way to do it.

This is effectively a revert of r223805.

llvm-svn: 263498