cpufeatures: Detecting support for `neon`?

[VorpalBlade]

It would be useful to be able to detect support for the neon instruction set on ARM, allowing to use vectorised instructions. As far as I can tell from the documentation, this is not yet supported.

[tarcieri]

Re: ARM, we only currently support AArch64, where the neon target feature is always enabled

[VorpalBlade]

Ah, so useless for me, I'm working on 32-bit ARM MCUs.

[tarcieri]

MCUs are also a bit of a different use case from our other ARM targets, where CPU feature detection is a privileged operation and we have to ask the OS about supported features through userspace APIs.

We can potentially support calling the privileged instructions (along with ARM32 targets in general), similar to how CPUID is implemented on x86 targets, but it's a bit different from what we've done for other targets.