Macho-embedded: update build script for newer Clang behaviour.

    
Clang now uses the *-apple-macho triples for embedded targets using MachO. The
environment makes less pretences to be running an actual Mach kernel with all
the support that would imply, so a few changes are needed to the build scripts:
    
+ The Apple way of detecting endianness applies in all cases so drop the
  __MACH__ test.
+ A quick rename since "darwin_embedded" no longer makes any sense. If there's
  no Mach going on, there's certainly no Darwin.
+ Change how we test for support for a platform. The functions we build don't
  need any SDK support (stdio.h, ...) so we don't need to check for it.

llvm-svn: 199731