[libc] Add aliases to C memory functions for integration tests

The integration tests require the C memory functions as the compiler may
emit calls to them directly. The tests normally use the `__internal__`
variant that is built for testing, but these memory functions were
linked directly to preserve the entrypoint. Instead, we forward delcare
the internal versions and map the entrypoints to them manually inside
the integration test. This allows us to use the internal versions of
these files like the rest of the test objects.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D146177