This is the first in a series of patches to turn the MachineOutliner on by default under -Oz for AArch64.
Currently, TargetMachines only know about -O0...-O3. They don't know anything about -Oz or -Os, and size optimization is managed solely by function attributes.
The outliner should be added only at -Oz under only AArch64 by default. With the way things currently work, that would mean the outliner, by default, would have to walk over every function in the module all of the time, and see if any minsize functions exist. This would incur a completely unnecessary compile-time overhead. Thus, this patch teaches Target Machines about size optimization levels.