Use LLVM libunwind for stack unwinding on Unix

[janvorli]

This change replaces external libunwind usage on Unix (including OSX) by a copy of the LLVM libunwind.
It also modifies libunwind to support reporting register location and implements
the unw_get_save_loc API to get this location.
For now, only memory locations are supported (register locations are not).
The support for register locations was added to x86, x64, arm and arm64. The OpenRISC and
PPC code was not modified.

[janvorli]

@jkotas, @sergiy-k can you take a look please?
Please note there are two commits - first one just brings in the unmodified libunwind sources, the other contains all the changes in our stuff and the libunwind stuff. So they should be merged in separately to preserve the history.

[janvorli]

There is something strange going on. I can build it on my OSX / Linux devices, but the CI build has failed. I have to investigate.

[jkotas]

I would be nice to have the libunwind sources in a commit that has nothing but the libunwind sources in it, set the author of the commit to dotnet-bot <[email protected]> and include link to the exact stamp from where it was forked in the commit description.

[janvorli]

The first commit is just that - only the author and stamp are not that way. Let me see if I can modify it that way.

[janvorli]

@jkotas I've modified the first commit that way.

[jkotas]

The first commit is just

It also has changes to integrate it into the corert build system and PAL. It would be nice to separate those out in a different commit.

[janvorli]

Hmm, I was sure I have separated that. Sigh.

[jkotas]

We should be able to remove libunwind8-dev from Documentation\prerequisites-for-building.md.

[janvorli]

Good point

[janvorli]

@jkotas I have moved stuff between commits using some git magic, so now the first commit should be pure import of the LLVM libunwind.

[janvorli]

I've also updated the prereq doc.

[jkotas]

Could you please also get in touch with llvm libunwind maintainers whether they would be open to upstreaming the change you made?

[jkotas]

#2166 can be reverted with this change.

[janvorli]

@jkotas I have already removed the #2166 as part of this change.

[jkotas]

Actually, I have done in in #2294. It is taken care of either way.