cpu: add support for DIT detection

Change-Id: I9f0991da9684ebd297e40a754e34130c418c34d0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/597377
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

cpu/cpu.go

@@ -105,6 +105,7 @@ var ARM64 struct {
 	HasSVE      bool // Scalable Vector Extensions
 	HasSVE2     bool // Scalable Vector Extensions 2
 	HasASIMDFHM bool // Advanced SIMD multiplication FP16 to FP32
+	HasDIT      bool // Data Independent Timing support
 	_           CacheLinePad
 }