[cxx] Make the externC of icalls configuration-dependent instead of always. (mono...

[cxx] Make the externC of icalls configuration-dependent instead of always. (mono/mono#17573)

There are pluses and minuses here:

 + Minimize externC.

 + Maximimze typesafe linkage.

 - Minimize configuration-sensitivity.

 - Maximize commonality across all configurations, so that
   success in one configuration maximally but not 100% implies
   success in others (which is why to avoid `#if` in general,
   to contribute toward compilation success).

 + Use higher level macros to explain reason.
   Even if ICALL_EXTERN_C was always externC (presently not),
   using it instead of plain G_EXTERN_C offers a builtin comment.

Commit migrated from https://github.com/mono/mono/commit/5237696f65cda0f079562838990b4effd4d65eee