make flang a cross-compiler

[h-vetinari]

From discussion in #61; this already came up in #28, where we descoped this at the time. This question is about how to do this practically.

Main question I see is how we want to name the feedstocks:

1. flang-feedstock for implementation; flang-activation-feedstock for activation
2. flang-impl-feedstock for implementation; flang-feedstock for activation

The GCC compilers follow more 1. (ctng-compilers & ctng-compiler-activation), but gfortran on osx follows more 2. (gfortran_impl_osx-64 & gfortran_osx-64). I don't really mind either way.

It would probably be convenient to do this as of flang 19, because then we don't have to deal with fortran_main (llvm/llvm-project@8d53866), but if we could do 18 already, that would probably help the emscripten-folks.

Any preferences for naming @isuruf?

[isuruf]

I prefer 1.

[h-vetinari]

OK, thanks for the quick response. Do you have a preference to try 18 first, or go straight to 19?

[isuruf]

Let's go straight to 19

[h-vetinari]

Got a PR to staged recipes: conda-forge/staged-recipes#27470