Compare real kinds directly

[certik]

In order to determine if the real kinds are equal, one can just compare them
directly, they are just integers. This is simpler, and also allows these lines
to be compiled with NAG. For some reason NAG does not seem to have c_sizeof()
as part of the iso_c_binding module and so it fails on these lines. This
commit fixes it.

[nncarlson]

The original code using c_size() is simply wrong (not just more complicated). amrex_real is an integer variable (or parameter), and c_size(amrex_real) is the C size of a Fortran integer, not the size of a real(kind=amrex_real) variable which was intended. The pull request code does exactly the right thing.

If there is any similar use of c_size() in the code, it's wrong and needs to be fixed.

[WeiqunZhang]

You are absolutely right.

[WeiqunZhang]

Fortunately, that's the only place c_sizeof is used.

[certik]

Thanks for merging @WeiqunZhang!