Bind(C): Fix type encodings in ISO_Fortran_binding.h
ISO_Fortran_binding.h had many incorrect hardwired kind encodings in
the definitions of the CFI_type_* macros. Additionally, not all
targets support all the defined type encodings, and the Fortran
standard requires those macros to have a negative value.
This patch changes ISO_Fortran_binding.h to use sizeof instead of
hard-coded sizes, and assembles it from fragments that reflect the
set of types supported by the target.