[libunwind] Take 2: Use the from-scratch testing configuration by default

Summary:
This commit switches libunwind from using the complicated logic in
libc++'s testing configuration to a from-scratch configuration.
I tried to make sure that all cases that were handled in the old
config were handled by this one too, so hopefully this shouldn't
break anyone. However, if you encounter issues with this change,
please let me know and feel free to revert if I don't reply quickly.

This change was engineered to be easily revertable.

This commit is a re-application of 5a8ad80b, which was reverted in
070a2ddc because it broke the Bootstrapping build. This has now been
addressed by tweaking the from-scratch config.

Differential Revision: https://reviews.llvm.org/D112082