[fortran] Add runtime warning mechanism to libgfortran

[FX Coudert]

Attached patch adds a new mechanism of runtime warnings and errors in 
libgfortran. This enables us to warn/error about non-standard complying 
constructions and extensions that can only be detected at runtime (such 
as format descriptor extensions) when -pedantic is specified during the 
compilation of the main program. The first such case is the $ edit 
descriptor:

malo ~/gfortran/debug $ cat dollar2.f
       character*5 c
       open (42,status='scratch')
       write (42,'(A,$)') 'abc'
       write (42,'(A)') 'de'
       rewind (42)
       read (42,'(A)') c
       close (42)

       if (c /= 'abcde') print *, 'ERROR'
       end
malo ~/gfortran/debug $ gfc dollar2.f
malo ~/gfortran/debug $ ./a.out
malo ~/gfortran/debug $ gfc -std=f95 -pedantic dollar2.f
malo ~/gfortran/debug $ ./a.out
At line 3 of file dollar2.f
Fortran runtime error: Extension: $ descriptor


Bootstrapped and regtesting in progress on i686-linux and x86_64-linux. 
OK for mainline (and 4.0 after testing)?

FX


:ADDPATCH fortran:
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