[BPF] add allows-misaligned-mem-access target feature

[clairechingching]

/cherry-pick a4a645f

I'd like to backport this change to handle misaligned memory access in the BPF target which was merged in this original PR. Backporting it so I can enable this feature in the rust nightly compiler

[clairechingching]

@yonghong-song I'd like to backport this change so that I can enable misalignment in the rust nightly compiler, thanks!

[yonghong-song]

I have no objection. But in the above, I see

Merging is blocked
    Cannot update this protected ref.

Not sure what is the problem.

[clairechingching]

@eddyz87 @c-rhodes could you take a look and merge this?

[c-rhodes]

@clairechingching backports are typically done via the /cherry-pick <commit> command left as a comment on the original PR, it's documented here: https://llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches

although I would say it's unlikely this will get backported so late in the release cycle given it's a feature. The next release is 21.1.7 on Dec 2nd, at this point in the release cycle the criteria is critical bug fixes as documented here https://llvm.org/docs/HowToReleaseLLVM.html#release-patch-rules. @tru is the release manager for 21.1.7, so ultimately it will be his decision. @tru wdyt?

[llvmbot]

@clairechingching backports are typically done via the /cherry-pick <commit> command left as a comment on the original PR, it's documented here: https://llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches

although I would say it's unlikely this will get backported so late in the release cycle given it's a feature. The next release is 21.1.7 on Dec 2nd, at this point in the release cycle the criteria is critical bug fixes as documented here https://llvm.org/docs/HowToReleaseLLVM.html#release-patch-rules. @tru is the release manager for 21.1.7, so ultimately it will be his decision. @tru wdyt?

Error: Command failed due to missing milestone.

[c-rhodes]

oops, apologies I didn't mean to trigger the bot, please ignore that.

[clairechingching]

@c-rhodes thanks for the clarification!

@tru

This change is a feature that doesn’t alter the default behavior, it simply enables misalignment support for the BPF target when explicitly requested. We’d like to backport it so that the Rust compiler can take advantage of this functionality. I’ve tested locally to confirm that the feature is correctly enabled when used.

Would appreciate it if you could accept the merge.