libstdc++: Declare malloc for freestanding

libstdc++: Declare malloc for freestanding

For a target with none of aligned_alloc, memalign etc. we defined our
own aligned_alloc using malloc, so we need a declaration of malloc. As
in libsupc++/new_op.cc we need to declare it ourselves for freestanding
environments.

libstdc++-v3/ChangeLog:

	* libsupc++/new_opa.cc [!_GLIBCXX_HOSTED]: Declare malloc.