[C++] Add support to GetRuntimeInfo for reporting ASIMD/NEON

[amoeba]

Describe the enhancement requested

PyArrow and R arrow have user-facing functions that call into Arrow C++'s arrow::GetRuntimeInfo() to report on build and runtime SIMD support:

arrow/cpp/src/arrow/config.cc

Lines 74 to 80 in 4a4d580

	RuntimeInfo GetRuntimeInfo() {
	RuntimeInfo info;
	auto cpu_info = CpuInfo::GetInstance();
	info.simd_level =
	MakeSimdLevelString([&](int64_t flags) { return cpu_info->IsSupported(flags); });
	info.detected_simd_level =
	MakeSimdLevelString([&](int64_t flags) { return cpu_info->IsDetected(flags); });

GetRuntimeInfo calls into MakeSimdLevelString to populate a RuntimeInfo object.
On ARM-based systems, "none" is reported for both SIMD levels which I think is caused by a there being no branch for CpuInfo::ASIMD:

arrow/cpp/src/arrow/config.cc

Lines 52 to 64 in f710ac5

	std::string MakeSimdLevelString(QueryFlagFunction&& query_flag) {
	if (query_flag(CpuInfo::AVX512)) {
	return "avx512";
	} else if (query_flag(CpuInfo::AVX2)) {
	return "avx2";
	} else if (query_flag(CpuInfo::AVX)) {
	return "avx";
	} else if (query_flag(CpuInfo::SSE4_2)) {
	return "sse4_2";
	} else {
	return "none";
	}
	}

1. Should we add a branch for CpuInfo::ASIMD?
2. If so, would it make more sense to call it ASIMD or NEON? They're related but slightly different concepts and I'm not sure what language users will be most familiar with and which makes more sense alongside other values.

Component(s)

C++

[mapleFU]

Nice catch! Also cc @cyb70289 for that. Do we have some dynamically enabled instr here?

[cyb70289]

Nice catch! Also cc @cyb70289 for that. Do we have some dynamically enabled instr here?

Looks no dynamically chosen code for Arm now. But I agree we should add NEON.

[amoeba]

Thanks! I'll file a PR later today.

[amoeba]

I re-opened this but I'm going to close it and file a new issue since this one is too narrow.