Polyfill for missing SIMD intrinsics in `cross-rs` image for target `aarch64-unknown-linux-musl`

A transitive dependency of spin (llama.cpp via ggml) uses the vld1q_s8_x4 and vld1q_u8_x4 compiler built-in SIMD intrinsics. These intrinsics are missing for aarch64 in gcc < 10.3, while cross-rs ships with gcc 9 for aarch64-unknown-linux-musl as of writing.

The code in this folder does a feature check and patches the arm_neon.h header with polyfills if the functions are missing.