Currently this is mostly a verbatim copy of macOS guide with some obvious changes.
Exact lists of dependencies for Debian and Arch need to be specified.
Also made minor fixes to Windows and macOS guides:
- changed http urls to https
- added --prefix=$PWD to crosstool-NG's configure
- added chmod u+w for the build output directory
In general, except for prerequisites, Linux and macOS guides are virtually identical.
It might make sense to refactor this into single document with prerequisites being
described in separate docs.
