[gfortran] Add malloc and free intrinsics

[François-Xavier Coudert]

> Shouldn't you make sure in gfc_check_{malloc,free} that the pointers
> are Cray pointers?

No. Compilers allow things like:

      integer i,ptr
      real*8 x(*)
      pointer(ptr,x)

      i = malloc(42)
      ptr = i
      x(0) = 17
      x(1) = -12
      i = ptr
      call free(ptr)
      end

> +@item @var{PTR} @tab The type shall be @code{INTEGER} with intent in.
>
> I don't know if we use this style throughout, but @code{INTENT(IN)}
> is probably prettier.  The documentation otherwise looks brief but
> correct.

OK. Will correct in the final commit, once approved.