Allow subtarget selection of the default MachineScheduler and document the interface.

The global registry is used to allow command line override of the
scheduler selection, but does not work well as the normal selection
API. For example, the same LLVM process should be able to target
multiple targets or subtargets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191071 91177308-0d34-0410-b5e6-96231b3b80d8