[runtimes][ci] Run the tests for libunwind in the CI

We should arguably have always been doing that. The state of libunwind
is quite sad, so this commit adds several XFAILs to make the CI pass.
We need to investigate why so many tests are not passing in some
configurations, but I'll defer that to folks who actually work on
libunwind for lack of bandwidth.

Differential Revision: https://reviews.llvm.org/D110872