MachO: use *-*-*-macho for MachO embedded targets.

Previously we had bodged together some hacks mapping MachO embedded
targets (i.e. mainly ARM v6M and v7M) to the "*-*-darwin-eabi" triple.
This is incorrect in both details (they don't run Darwin and they're
not EABI in any real sense).

This commit appropriates the existing "MachO" environment for the
purpose instead.