The pause instruction requires SSE2 but was being unconditionally used
on targets without it, resulting in undefined behavior.

This PR fixes that by only using the pause intrinsic if SSE2 is available.

It also removes the inline assembly which was not required since these
instructions are available in core::arch, and extends support of
the spin_loop hint to arm targets with the v6 feature which also
support the yield instruction.

Closes rust-lang#59237 .

Since commit 9452a8d, the new debug info format is only generated
for LLVM 8 and newer versions. However, the tests still assume that LLVM
7 will use the new debug info format. Fix the tests (and a comment in
the code) to match the actual version check.

Track changes to robots.txt

Currently `robots.txt` of doc.rust-lang.org is not part of any repo, so there's [no way to contribute any changes to it](https://internals.rust-lang.org/t/deadlock-about-fixing-outdated-documentation-links-in-search-engines/9374), such as needed for rust-lang#44894 and countless dupes of this issue.

I propose adding it to this repo. I'm not in control of the infrastructure, so I can't help to automate deployment of it, but even just having the file under source control is IMHO a step forward.

Remove inline assembly from hint::spin_loop

This PR removes the inline assembly which was not required since these
instructions are available in core::arch, and extends support of
the spin_loop hint to arm targets with the v6 feature which also
support the yield instruction.

…chenkov

Use a valid name for graphviz graphs

Hiridification has broken graphviz output because `HirId` has a more complex display implemetation than `NodeId`. Since the id was just used to generate a distinct identifier, we just pull out the various constituent indexed.

Do not encode gensymed imports in metadata

(Unless they are underscore `_` imports which are re-gensymed on crate loading, see rust-lang#56392.)

We cannot encode gensymed imports properly in metadata and if we encode them improperly, we can get erroneous name conflicts downstream.
Gensymed imports are produced by the compiler, so we control their set, and can be sure that none of them needs being encoded for use from other crates.

A workaround that fixes rust-lang#59243.

Implement specialized nth_back() for Box and Windows.

Hi there, this is my first pull request to rust :-)

I started implementing some specializations for DoubleEndedIterator::nth_back() and these are the first two. The problem has been discussed in rust-lang#54054 and nth_back() is tracked in rust-lang#56995.

I'm stuck with the next implementation so I though I do a PR for the ones I'm confident with to get some feedback.

…trochenkov

Fix ICE with const generic param in struct

Fixes rust-lang#59340.

r? @petrochenkov

Correct minimum system LLVM version in tests

Since commit 9452a8d, the new debug info format is only generated
for LLVM 8 and newer versions. However, the tests still assume that LLVM
7 will use the new debug info format. Fix the tests (and a comment in
the code) to match the actual version check.