Mark avx broadcast intrinsics as safe

[okaneco]

The scalar functions are wrappers around _mm_set1_* functions which are already safe.

_mm256_broadcast_ps and _mm256_broadcast_pd take their input argument by reference instead of raw pointer.

[rustbot]

r? @Amanieu

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[sayantn]

The CI failure seems strange, could you force push it once to retry the CI? You can probably just do git commit --amend --allow-empty --no-edit && git push --force if the worktree is clean

[okaneco]

CI failed twice on Run ./ci/run-docker.sh armv7-unknown-linux-gnueabihf.

When I first submitted the PR, the failing x86_64-unknown-freebsd didn't seem to even start so it timed out.

[sayantn]

The ARM failures bisect to rust-lang/rust#142768
I am not sure why the freebsd run didn't start, probably spurious since it didn't happen this time