Embedded development often needs to use a different C standard library,
replacing the existing one normally passed as -internal-externc-isystem.
This works fine for an apple-macos target, but apple-none-macho doesn't
work because the MachO driver doesn't implement
AddClangSystemIncludeArgs to add the resource directory as
-internal-isystem like most other drivers do. Move most of the search
path logic from Darwin and DarwinClang down into an AppleMachO toolchain
between the MachO and Darwin toolchains.
Also define __MACH__ for apple-none-macho, as Swift expects all MachO
targets to have that defined.
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