Implement vector element extensions for AArch64

This commit also includes load and extend operations. Both are
prerequisites for enabling further SIMD spec tests.

Copyright (c) 2020, Arm Limited.

build.rs

@@ -180,8 +180,8 @@ fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
             _ => (),
         },
         "Cranelift" => match (testsuite, testname) {
-            ("simd", "simd_store") => return false,
             ("simd", "simd_i8x16_cmp") => return false,
+            ("simd", "simd_store") => return false,
             // Most simd tests are known to fail on aarch64 for now, it's going
             // to be a big chunk of work to implement them all there!
             ("simd", _) if target.contains("aarch64") => return true,