Support wasm select instruction with V128-typed operands on AArch64.

* this requires upgrading to wasmparser 0.67.0.

* There are no CLIF side changes because the CLIF `select` instruction is
  polymorphic enough.

* on aarch64, there is unfortunately no conditional-move (csel) instruction on
  vectors.  This patch adds a synthetic instruction `VecCSel` which *does*
  behave like that.  At emit time, this is emitted as an if-then-else diamond
  (4 insns).

* aarch64 implementation is otherwise straightforwards.

cranelift/filetests/filetests/isa/aarch64/simd.clif

@@ -47,3 +47,17 @@ block0:
 ; nextln:  mov sp, fp
 ; nextln:  ldp fp, lr, [sp], #16
 ; nextln:  ret
+
+function %f4(i32, i8x16, i8x16) -> i8x16 {
+block0(v0: i32, v1: i8x16, v2: i8x16):
+   v3 = select v0, v1, v2
+   return v3
+}
+
+; check:  stp fp, lr, [sp, #-16]!
+; nextln:  mov fp, sp
+; nextln:  subs wzr, w0, wzr
+; nextln:  vcsel v0.16b, v0.16b, v1.16b, ne (if-then-else diamond)
+; nextln:  mov sp, fp
+; nextln:  ldp fp, lr, [sp], #16
+; nextln:  ret