Leverage target_feature 1.1

[lu-zero]

target_feature 1.1 should ease implementing some SIMD code.

• safe #[target_feature] functions can be called without an unsafe {} block only from functions that have at least the exact same set of #[target_feature]s. Calling them from other contexts (other functions, static variable initializers, etc.) requires opening an unsafe {} even though they are not marked as unsafe
• safe #[target_feature] functions are not assignable to safe fn pointers.

This issue is a tracker to know when we'll be able to use it.

[vibhoothi]

I think we are now using this for NEON, AVX2 post-dav1d merge, can be closed I think

[lu-zero]

Not yet rust-lang/rust#69098