cpu: don't panic on error reading /proc/self/auxv

On Android /proc/self/auxv is not readable, leading to a panic in init.
Instead of panic'ing in this case, introduce the Initialized global bool
to report whether the CPU features were initialized i.e. /proc/self/auxv
was successfullly read.

Fixes golang/go#30413

Change-Id: Ib520f202990614a76bceb0bf9d19a88eadd13e10
Reviewed-on: https://go-review.googlesource.com/c/164057
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

cpu/cpu_test.go

@@ -13,6 +13,9 @@ import (
 
 func TestAMD64minimalFeatures(t *testing.T) {
 	if runtime.GOARCH == "amd64" {
+		if !cpu.Initialized {
+			t.Fatal("Initialized expected true, got false")
+		}
 		if !cpu.X86.HasSSE2 {
 			t.Fatal("HasSSE2 expected true, got false")
 		}