[libc] Enable the GPU build to build the UnitTest library

The previous patch in D149216 allows us to use the internal `<stdlib.h>`
include for the GPU build. However, we currently don't provide the
memory functions so the header wasn't resolving them. This patch adds
these as entrypoints. They don't cause any entrypoints to be emitted
because they are not implemented, but they provide it in the header so
that we can rely on the test's implementation of them.

Depends on D149216

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149217