Rename getPersonalityPICSymbol to getCFIPersonalitySymbol, document it, and

give it a bit more responsibility. Also implement it for MachO.

If hacked to use cfi, 32 bit MachO will produce

.cfi_personality 155, L___gxx_personality_v0$non_lazy_ptr

and 64 bit will produce

.cfi_presonality ___gxx_personality_v0

The general idea is that .cfi_personality gets passed the final symbol. It is
up to codegen to produce it if using indirect representation (like 32 bit
MachO), but it is up to MC to decide which relocations to create.

llvm-svn: 130341