Add a target asm info hook to specify that particular bits of data in the FDE

Add a target asm info hook to specify that particular bits of data in the FDE
should be forced to 32-bits (.long) even on 64-bit architectures. Darwin wants
these bits to be 64-bits (.quad). However, other platforms may disagree.

This is just the info right now and is part of a work-in-progress which needs
this. We'll add the actual *use* of this soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@80024 91177308-0d34-0410-b5e6-96231b3b80d8