Loadable targets via compiler plugins

[mahkoh]

This patch adds support for the PPC64 target and is quite small. Maybe those things should be moved out of the compiler and into dynamically loaded libraries. This would allow people to use targets that are not currently supported by the compiler.

[bjorn3]

We have had unstable support for specifying target specifications using external json files for years now. As example on how to use them see https://os.phil-opp.com/minimal-rust-kernel/#target-specification