Build script for klepto toolchain

You need Python as this build script and the Swift build script and the SwiftPM build script are all written in Python.

You need a host Swift toolchain of the same version as the klepto toolchain to build ̀ klepto-swiftpm`. You also need CMake, and all other dependencies of a regular Swift toolchain build.

You also obviously need a devkitPro environment setup with devkitA64 and libnx. The DEVKITPRO environment variable must be set.

1. Clone every repository in the organization next to each other
2. Go to the folder containing everything and run the build script python3 klepto-build/build.py
3. If it whines that something is missing, which it probably will because I didn't fork everything, clone it from the corresponding upstream Swift repository and checkout the right branch (same branch name as the swift repository)

There are options to build individual parts of the toolchain, create a targz package, make a dry run... Use python3 klepto-build/build.py --help to list them.