ARM: make sure backend is consistent about exception handling method.

It turns out we decide whether to use SjLj exceptions or some alternative in
two separate places in the backend, and they disagreed with each other. This
led to inconsistent code and is generally a terrible idea.

So make them consistent and add an assert that they *do* match (unfortunately
MCAsmInfo isn't available in opt, so it can't be used to initialise the CodeGen
version directly).

llvm-svn: 253502