Document how to build higan/byuu on each supported platform

[Screwtapello]

The Cirrus CI scripts technically record all the required information, but it would be nice to have the same information in a more human-friendly format.

This should include how to make a debug build, and possibly also how to gather a backtrace, because that's going to come up a lot.

Issue #31 is related to this.

[AndersonTorres]

Repology accuses a new version of Higan, but distros are stuck in v110.

The new higan is so different from the older version that the old scripts can't be reused, at least not without big changes.

For now I am studying the building system here.