Fix for failure to unwind Linux stack frames with call in final position.

Fixes http://llvm.org/bugs/show_bug.cgi?id=18656.

Note this exposes a failure on Linux of
TestInferiorAssert.test_inferior_asserting_disassemble, similar to how
it fails on FreeBSD. I'll file a bug for this next. We're now getting
another frame beyond where we used to prior to this fix, so the fix is
exposing failures in previosly not-reachable frames.

Much thanks to Jason Molenda, who had much to do with helping figure
out where unwinding was breaking.