[libc] Add the 'cpp.new' as a dependency on `atexit`

The `atexit` function depends on the implementations in CPP/new.h but it
is not listed as a dependency. This causes the GPU build to not include
it in the `libcgpu.a` file and prevents us from using the startup code
externally. Simply add it.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D158447