Branch island debug (#139166)
This patch allows lldb to step in across "branch islands" which is the
Darwin linker's way of dealing with immediate branches to targets that
are too far away for the immediate slot to make the jump.
I submitted this a couple days ago and it failed on the arm64 bot. I was
able to match the bot OS and Tool versions (they are a bit old at this
point) and ran the test there but sadly it succeeded. The x86_64 bot
also failed but that was my bad, I did @skipUnlessDarwin when I should
have done @skipUnlessAppleSilicon.
So this resubmission is with the proper decoration for the test, and
with a bunch of debug output printed in case of failure. With any luck,
if this resubmission fails again I'll be able to see what's going on.
