[machinst x64]: enable packed arithmetic spec tests

This change should finish off the implementation of packed arithmetic in the new backend.

build.rs

@@ -183,8 +183,11 @@ fn experimental_x64_should_panic(testsuite: &str, testname: &str, strategy: &str
         ("simd", "simd_address") => return false,
         ("simd", "simd_const") => return false,
         ("simd", "simd_i8x16_arith") => return false,
+        ("simd", "simd_i8x16_arith2") => return false,
         ("simd", "simd_i16x8_arith") => return false,
+        ("simd", "simd_i16x8_arith2") => return false,
         ("simd", "simd_i32x4_arith") => return false,
+        ("simd", "simd_i32x4_arith2") => return false,
         ("simd", "simd_i64x2_arith") => return false,
         ("simd", "simd_f32x4_arith") => return false,
         ("simd", "simd_f32x4_cmp") => return false,