cpu: support reading arm64 CPU feature registers

This allows to detect ARM64 CPU features on non-Linux systems. On Linux,
this is used in case /proc/self/auxv cannot be read.

Change-Id: I67d55e989f2beda0c05a97ca5e55781840a8e01c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/209478
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_linux_arm64.go

@@ -4,8 +4,6 @@
 
 package cpu
 
-const cacheLineSize = 64
-
 // HWCAP/HWCAP2 bits. These are exposed by Linux.
 const (
 	hwcap_FP       = 1 << 0
@@ -35,6 +33,12 @@ const (
 )
 
 func doinit() {
+	if err := readHWCAP(); err != nil {
+		// failed to read /proc/self/auxv, try reading registers directly
+		readARM64Registers()
+		return
+	}
+
 	// HWCAP feature bits
 	ARM64.HasFP = isSet(hwCap, hwcap_FP)
 	ARM64.HasASIMD = isSet(hwCap, hwcap_ASIMD)