Add handling for restartable sequence corner cases

[derekbruening]

This covers handling some corner cases for restartable sequences ("rseq") which our original implementation for #2350 does not handle.
These are from the list at #2350 (comment)

There are a number of corner cases left to cover, but they are lower priority. These are things like:

• Invoking the 2nd execution on a sequence midpoint exit (maybe leveraging __rseq_exit_point_array)
• Handling indirect branch exits out of a sequence
• Remove the -rseq_assume_call option once we're satisfied we'll never go back to it
• Add more sanity checks for existing requirements/assumptions