[release/8.0-preview4] Comment out support for mtag extension in libunwind

[jkotas]

Fixes dotnet/source-build#3424

Customer Impact

Break in source build caused by recent upgrade of libunwind from libunwind upstream. The update brought it inline assembly code that does not compile with older compilers.

Comment out the offending code since it is not needed for our libunwind use case.

Testing

Local build

Risk

Low. The change is effectively reverting recent change.

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Issue Details

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Author:	jkotas
Assignees:	jkotas
Labels:	area-Infrastructure-coreclr, needs-area-label
Milestone:	-

[jkotas]

cc @mthalman

[VSadov]

Regarding:

Please reference the commit modifying external source in src/native/external/llvm-libunwind-version.txt.

I assume we will do a similar change in the main and then reference that commit?

@am11

[VSadov]

LGTM Thanks!

[am11]

I assume we will do a similar change in the main and then reference that commit?

We can reference the commit during the forwardport to main branch (makes it easier during the updates).

[jeffschwMSFT]

@jkotas @VSadov this is ready to merge. Please take a look at the CI failures.

[VSadov]

It looks like the PR needs rebasing

[jeffschwMSFT]

@VSadov thanks for taking a look. To make Preview 4 we will need to merge today. Hopefully we will have a green ci by then, but we will need to monitor it.

[VSadov]

@jeffschwMSFT I think the failures were unrelated to the change, since this is a build fix that is specific to nativeaot+arm64+linux, but the PR got behind the base branch and became unmergeable (according to GitHub).
I've rebased the PR.

I think once NativeAOT builds on linux/arm64, we will know that the fix works (there should be no other effects), but it would be better to see green where it can be green. I'll be watching.

[mthalman]

FWIW, I've verified this fix in the pipeline that originally found this issue: Debian11_Offline_MsftSdk_arm64 (internal link). This build leg was previously failing (internal link) without this change.