[patch,fortran] Support type real for count_rate in system_clock (PR28484)

[François-Xavier Coudert]

> Are you aware of http://gcc.gnu.org/PR15516 ?
> "assembly snippets for nano second resolution wall clock time"
>
> Is this something we want to have?

I think it would be nice to have it, and it sure will be a very good
initiation to autoconf :)

But perhaps do it in two steps: the system_clock intrinsic with
integer/real arguments of all kinds, and then the nanosecond
resolution.

FX