Enable simd_extmul_* for AArch64

Lower simd_extmul_[low/high][signed/unsigned] to [s|u]widen inputs to
an imul node.

Copyright (c) 2021, Arm Limited.

build.rs

@@ -225,7 +225,6 @@ fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
             // These are new instructions that are not really implemented in any backend.
             ("simd", "simd_i16x8_extadd_pairwise_i8x16")
             | ("simd", "simd_i32x4_extadd_pairwise_i16x8") => return true,
-
             _ => {}
         },
         _ => panic!("unrecognized strategy"),