Add iabs instructions.rs

This is necessary for implementing SIMD [abs](https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#lane-wise-integer-absolute-value).

cranelift/codegen/meta/src/shared/instructions.rs

@@ -2347,6 +2347,18 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    ig.push(
+        Inst::new(
+            "iabs",
+            r#"
+        Integer absolute value with wrapping: `a := |x|`.
+        "#,
+            &formats.unary,
+        )
+        .operands_in(vec![x])
+        .operands_out(vec![a]),
+    );
+
     ig.push(
         Inst::new(
             "imul",

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2069,6 +2069,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             panic!("x86-specific opcode in supposedly arch-neutral IR!");
         }
 
+        Opcode::Iabs => unimplemented!(),
         Opcode::AvgRound => unimplemented!(),
         Opcode::TlsValue => unimplemented!(),
     }