runtime: improve handling of panic during deferred function

runtime: improve handling of panic during deferred function

    When a panic occurs while processing a deferred function that
    recovered an earlier panic, we shouldn't report the recovered panic
    in the panic stack trace. Stop doing so by keeping track of the panic
    that triggered a defer, marking it as aborted if we see the defer again,
    and discarding aborted panics when a panic is recovered. This is what
    the gc runtime does.

    The test for this is TestRecursivePanic in runtime/crash_test.go.
    We don't run that test yet, but we will soon.

    Reviewed-on: https://go-review.googlesource.com/46461

From-SVN: r249590