[libc] Partially implement 'quick_exit' for the GPU

This patch adds a partial implementation of `quick_exit` for the GPU
target. This is mainly done to test object libraries for the GPU and
will be expanded later. This will simply cause the threads to terminate
on the GPU without returning an error code. This functionality will be
added later to facilitate unit tests.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D144421