[libc] Implement `exit` for the GPU partially

This patch implements the `exit` function on the GPU. This required
breaking the entrypoints calling eachother on `linux` since this doesn't
work with a non-aliased target. This is only partial support because
full support requires a malloc / free implementation for the exit
callbacks array.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149363