Add -foutline option to enable the MachineOutliner in AArch64

Since we've been working on productizing the MachineOutliner in AArch64, it
makes sense to provide a more user-friendly way to enable it.

This allows users of AArch64 to enable the outliner using -foutline instead
of -mllvm -enable-machine-outliner. Other, less mature implementations (e.g,
x86-64) can still enable the pass using the -mllvm option.

Also add a test to make sure it works.

llvm-svn: 331370