Draft: aarch64: add DAV1D_ARM_CPU_FLAG_DOTP

Runtime detection on Linux using HWCAP_CPUID for user space access to the CPU feature registers. See https://www.kernel.org/doc/html/latest/arm64/cpu-feature-registers.html

Runtime detection on MacOS using sysctlbyname("hw.cpufamily") and matching known ARMv8.4-A CPUs. MacOS unfortunately doesn't expose a flag for this feature.

marked as draft as it is pointless without users.