The GNU assembler expects `.size symbol, .-symbol`. The previous
`.size spend_cpu_cycles, spend_cpu_cycles` expression is not a constant
and breaks compilation on AArch64 Linux. Use the correct form so the
project builds on ARM64.
Replace the scalar ARM fallback in update_histogram_buckets with a NEON
implementation that processes two buckets per iteration, matching the
existing AVX path. The wrapper now dispatches to the SIMD path on both
x86-64 and AArch64 and falls back to scalar code on other architectures.
The metrics histogram update code unconditionally included <immintrin.h>
and used __attribute__((target("avx"))) SSE/AVX intrinsics, which only
exist on x86-64. This prevented the project from compiling on ARM64.
Guard the x86-64 SIMD implementation and the <immintrin.h> include with
an architecture check, and add a portable scalar fallback for non-x86-64
platforms (e.g., ARM64). A thin wrapper function keeps the call sites
unchanged and preserves the AVX fast path on x86-64.
Closes#3
weaselbot