ARM needs a "thumb-state" target-feature to fix a recent breakage that `stdsimd` caused.

[Lokathor]

So recently stdsimd added an intrinsic that was too liberal. The intrinsic needs to only be allowed in in specific circumstances, like so:

#[cfg(any(not(target_feature = "thumb-state"), target_feature = "v6t2"))]

Unfortunately we don't currently have a "thumb-mode" feature.

@gnzlbg said that before a PR for this change we should first have an issue with a ping to @parched, @Amanieu, and @japaric , so that everyone can be on the same page about how to proceed.

EDIT: Within official ARM docs, using ARM or Thumb code is called the "state", not "mode", so we should stick with that naming system. The CPU's "mode" is reserved for a different concept.

[CJKay]

This is critical for certain inline assembly like semihosting instructions. There is no way to identify that armv7-linux-androideabi is going to build with Thumb code generation but arm-unknown-linux-gnueabi isn't.

[jonas-schievink]

Related: rust-lang/rfcs#2867