We need to only generate references to `_swift_exceptionPersonality`
if we're building for a new enough runtime. The previous code was
good on Darwin, but would have resulted in build problems on Linux.
rdar://120952971
The other new runtime functions appear to have a leading underscore.
It makes sense in this case because we don't expect anything to call
this directly (other than the unwinder).
rdar://120952971
The exception handling tests should check for the new personality, and
the ABI check needs to know about the new entry point in the runtime.
rdar://120952971
