Add builtin compilers and architecture targets #13
Description
Instead of having to redefine for every target common settings like compiler, sysroot, system includes, target triplet, etc, create builtin configurations that can be configured with a variable such as LOCAL_BUILTIN_COMPILER and LOCAL_BUILTIN_ARCH
It would be even nicer to allow the user to create their own customized targets as well and use them with these variables if the builtins are not good enough or need customization.