Toolchains 20221025

LLVM upgraded to 15.0.3.

Toolchains 20220513

toolchain-bootstrap: copy all libraries into output artifact

We didn't copy some libraries like libatomic, which are part of the
base GNU toolchain and should always be available.

Toolchains 20220508

toolchain-bootstrap: add project for building an LLVM toolchain

The goal of this project is to build highly portable, generic
toolchains.

This project is a spiritual successor to functionality I built for
python-build-standalone. An immediate goal is to rip out the toolchain
building from that project and have it consume pre-built toolchains
from this project.

I've chosen Earthly to help orchestrate all the building because the
DSL is highly expressive.