Driver: make it easier to select the SjLj EH model

GCC still permits enabling the SjLj EH model.  This is something which can be
done on various targets.  Hoist the -fsjlj-exceptions option into the driver and
pass it through.  This allows one to opt into the alternative EH model while
retaining the default to be the target's default.

Resolves PR27749!

llvm-svn: 272424