[gfortran] Add malloc and free intrinsics

[FX Coudert]

> Unless some miracle happens RSN, it unfortunately looks like 4.1 is
> going to release without cray pointers. In that case, IMHO malloc/free
> can wait for 4.2 as well.

It's going to branch, but I'm not sure the policy after branching has 
been clearly set.  I certainly think it would be better for gfortran to 
try keeping 4.1 in sync as much as possible.

FX