Cross compiler support

Just some notes for myself.

Issues
1. ~Split out sysroot~
2. ~Split out crt files from gcc~
3. Build msitools
4. ~Package up https://gist.github.com/isuruf/2150a2b988c89d60df1fa81e527983eb~
5. Build binutils
6. Build cctools and ld64
7. Build clang
8. https://github.com/conda/conda-build/issues/3155
9. ~Split OpenMP headers~

| Build system| linux-64 | osx-64 | linux-ppc64le/aarc64 | win-64 |
| ------------- | ------------- | -----------|--------------| ---------|
| linux-64 | native | `clang_osx-64`, `gfortran_osx-64` | `gcc_linux-*`, `gfortran_linux-*` | `clang_win-64` |
| osx-64 | 1 & 2 & 5 | native | 1 & 2 & 5 | 3 & 4 |
| win-64 | 1 & 2 & 5 | 6 | 1 & 2 & 5 | native |