[Patch, Fortran] Implement F2008's c_sizeof (PR36437)

[Tobias Burnus]

This patch implements Fortran 2008's C_SIZEOF, which is a one liner. It 
additionally does:
- Add documentation (essentially a copy from SIZEOF)
- Fix keyword= should be x= and not i=
- Simplify the generated tree. For scalars gfortran produced:

  {
    integer(kind=8) bytes.1;
    i = 4;
  }

  and now it simpy generates:   i = 4;

Build and "make check-gfortran pdf html" tested on x86-64-linux. OK for 
the trunk?

Tobias
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