Target mapping

[pedrolamarao]

The original "psys" build system, based on nokee, allowed us to define custom targets.

This enabled us to define targets such as x86_32-elf-multiboot2, something that makes perfect sense for this project: a target that generates code for x86_32 and links an ELF Multiboot2 application. This top level target would configure compilation for i686-elf and would include a dependency on module multiboot2, perhaps even link the application as a module to a multiboot2 start main module.

We could still support such a system if we had some kind of target mapping, that would take the "top" target name and map that to compile target and link target names.

That would be immediately useful, since clang thinks it needs gcc to link target i686-elf and we must trick it to use lld by transforming that to i686-linux-elf. This transform is hard-coded in MetalLinkTask.