ARM: prefer allocating VFP regs at stride 4 on Darwin.

ARM: prefer allocating VFP regs at stride 4 on Darwin.

This is necessary for WatchOS support, where the compact unwind format assumes
this kind of layout. For now we only want this on Swift-like CPUs though, where
it's been the Xcode behaviour for ages. Also, since it can expand the prologue
we don't want it at -Oz.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243884 91177308-0d34-0410-b5e6-96231b3b80d8