cpu: get hwcap/auxv from the Go 1.21+ runtime

Depends on https://go.dev/cl/458256

This change only does Linux for now.

Updates golang/go#57336

Change-Id: I0659697c1bdc6e2577c6251b964a0df32047ee12
Reviewed-on: https://go-review.googlesource.com/c/sys/+/465295
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

cpu/hwcap_linux.go

@@ -24,6 +24,21 @@ var hwCap uint
 var hwCap2 uint
 
 func readHWCAP() error {
+	// For Go 1.21+, get auxv from the Go runtime.
+	if a := getAuxv(); len(a) > 0 {
+		for len(a) >= 2 {
+			tag, val := a[0], uint(a[1])
+			a = a[2:]
+			switch tag {
+			case _AT_HWCAP:
+				hwCap = val
+			case _AT_HWCAP2:
+				hwCap2 = val
+			}
+		}
+		return nil
+	}
+
 	buf, err := ioutil.ReadFile(procAuxv)
 	if err != nil {
 		// e.g. on android /proc/self/auxv is not accessible, so silently