Use a SmallVec for ABIArgSlots (#4586)

These are always length 1 for Wasm benchmarks.

<h3>Sightglass Benchmark Results</h3>

```
compilation :: nanoseconds :: benchmarks/spidermonkey/benchmark.wasm

  Δ = 328624015.86 ± 40274677.93 (confidence = 99%)

  main.so is 0.88x to 0.91x faster than slots-smallvec.so!
  slots-smallvec.so is 1.10x to 1.13x faster than main.so!

  [3070752447 3203778792.55 3446269274] main.so
  [2503544039 2875154776.69 3197966713] slots-smallvec.so

compilation :: nanoseconds :: benchmarks/pulldown-cmark/benchmark.wasm

  Δ = 9685705.06 ± 3221286.87 (confidence = 99%)

  main.so is 0.91x to 0.96x faster than slots-smallvec.so!
  slots-smallvec.so is 1.05x to 1.09x faster than main.so!

  [129356493 145594942.79 165038803] main.so
  [118555011 135909237.73 188780619] slots-smallvec.so

compilation :: nanoseconds :: benchmarks/bz2/benchmark.wasm

  No difference in performance.

  [79080493 86757564.46 112649639] main.so
  [78083384 85934125.69 94992743] slots-smallvec.so
```

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

@@ -212,7 +212,7 @@ impl ABIMachineSpec for AArch64MachineDeps {
                     let upper_reg = xreg(next_xreg + 1);
 
                     ret.push(ABIArg::Slots {
-                        slots: vec![
+                        slots: smallvec![
                             ABIArgSlot::Reg {
                                 reg: lower_reg.to_real_reg().unwrap(),
                                 ty: param.value_type,

cranelift/codegen/src/isa/x64/abi.rs

@@ -125,7 +125,7 @@ impl ABIMachineSpec for X64ABIMachineSpec {
                 );
             }
 
-            let mut slots = vec![];
+            let mut slots = ABIArgSlotVec::new();
             for (rc, reg_ty) in rcs.iter().zip(reg_tys.iter()) {
                 let intreg = *rc == RegClass::Int;
                 let nextreg = if intreg {