Specify minimum llvm version for aarch64 assembly test

Author: jhorstmann

tests/assembly/simd-intrinsic-mask-reduce.rs

@@ -4,6 +4,7 @@
 // [x86] needs-llvm-components: x86
 // [aarch64] compile-flags: --target=aarch64-unknown-linux-gnu
 // [aarch64] needs-llvm-components: aarch64
+// [aarch64] min-llvm-version: 15.0
 // assembly-output: emit-asm
 // compile-flags: --crate-type=lib -O