cpu: add DIT option and hwcap DIT support

Follow-up to https://go.dev/cl/597377, add a option for DIT so it can be
set via GODEBUG, and add hwcap support for Linux.

Change-Id: Ib094b520edea9c099d37f121b09d02b31644a433
Reviewed-on: https://go-review.googlesource.com/c/sys/+/598719
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

cpu/cpu_arm64.go

@@ -38,6 +38,7 @@ func initOptions() {
 		{Name: "dcpop", Feature: &ARM64.HasDCPOP},
 		{Name: "asimddp", Feature: &ARM64.HasASIMDDP},
 		{Name: "asimdfhm", Feature: &ARM64.HasASIMDFHM},
+		{Name: "dit", Feature: &ARM64.HasDIT},
 	}
 }