Add iOS and tvOS minimally supported intrinsics

[marek-safar]

Both iOS and tvOS have a well-known set of hardware where it can run. If we consider iOS10 and tvOS10 as minimal versions we could fill a minimally supported intrinsics in SPC substitutions to trim any software fallback.

Note: The same will probably apply to Apple Silicon once it's out

@EgorBo @rolfbjarne

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[EgorBo]

We currently don't have any arm64 intrinsics in Mono except https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Runtime/Intrinsics/Arm/ArmBase.cs which should be enabled by default for any arm64 hw.

Vector<T> should be, in theory, quite easy to implement since our LLVM-engine already emits cross-platform llvm vectors for it.

[marek-safar]

Yeah, we'll have to guard that for LLVM codegen only

[rolfbjarne]

If we consider iOS10 and tvOS10 as minimal versions

Aren't intrinsics usually based on the cpu and not the OS version?

[marek-safar]

@rolfbjarne true but I'm referring to OS version as the definition for the minimally supported hw/cpu.

[SamMonoRT]

@imhameed - if iOS uses AOT compilation with LLVM, all the work you did to add intrinsics is applicable here ? If so, please close the issue.

[SamMonoRT]

xamarin/xamarin-macios#11379 should fix this issue, and we can close this out.

[imhameed]

We should enable NEON by default for arm64 but yeah--I'll open a PR for this--all the arm64 hardware intrinsics ought to work on Mono.

[imhameed]

This was done in #56323.