add stateify savable tag for FeatureSet of Arm64

Signed-off-by: Howard Zhang <howard.zhang@arm.com>

pkg/cpuid/cpuid_arm64.go

@@ -31,6 +31,8 @@ import (
 // Documentation/arm64/elf_hwcaps.rst for more info.
 //
 // Currently, only the HWCAP bits are supported.
+//
+// +stateify savable
 type FeatureSet struct {
 	hwCap      uint
 	cpuFreqMHz float64