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_arm64.go

@@ -168,6 +168,11 @@ func parseARM64SystemRegisters(isar0, isar1, pfr0 uint64) {
 
 		parseARM64SVERegister(getzfr0())
 	}
+
+	switch extractBits(pfr0, 48, 51) {
+	case 1:
+		ARM64.HasDIT = true
+	}
 }
 
 func parseARM64SVERegister(zfr0 uint64) {