[fortran] Add runtime warning mechanism to libgfortran

[FX Coudert]

> I have reviewed this patch, it works, and I regtested it on FC3/Athlon.
> 
> I tried and partially succeded to write a testcase for it, based on FX's 
> example (see 5 below).
> 
> It's OK for mainline and 4.0, with a couple of remarks.

Thanks for the testcase and the reviewing. Attached is what I commited.

FX
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