Darwin-embedded: find correct libclang_rt for embedded targets.

This refactors some of the Darwin toolchain classification to give a more solid
distinction between the three primary Darwin platforms (OS X, IOS and IOS
simulator) so that a 4th choice can be added temporarily: embedded MachO
targets.

Longer term, this support will be factored out into a separate class and no
longer classified as "darwin-eabi", but the refactoring should still be useful.