Revert "[libc++] Move abs and div into stdlib.h to fix header cycle."

This reverts commit 82b47b29.

This broke Clang and LLDB module builds without -fmodules-local-submodule-visbility.
I'll revert this for now until we have a fix and reland once Clang
can properly handle this code.

See also the discussion in https://reviews.llvm.org/rG82b47b2978405f802a33b00d046e6f18ef6a47be