[MachineOutliner] Add support for target-default outlining.

This adds functionality to the outliner that allows targets to
specify certain functions that should be outlined from by default.

If a target supports default outlining, then it specifies that in
its TargetOptions. In the case that it does, and the user hasn't
specified that they *never* want to outline, the outliner will
be added to the pass pipeline and will run on those default functions.

This is a preliminary patch for turning the outliner on by default
under -Oz for AArch64.

https://reviews.llvm.org/D48776

llvm-svn: 336040