[libc] Add the implementation of the fflush function.

Note that the underlying flush implementation does not yet fully implement
the POSIX standard. It is complete with respect to the C standard
however. A future change will add the POSIX behavior. It should not affect
the implementation of the fflush function however as the POSIX behavior
will be added in a lower layer.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D124073