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_linux.go

@@ -28,7 +28,9 @@ var hwCap2 uint
 func init() {
 	buf, err := ioutil.ReadFile(procAuxv)
 	if err != nil {
-		panic("read proc auxv failed: " + err.Error())
+		// e.g. on android /proc/self/auxv is not accessible, so silently
+		// ignore the error and leave Initialized = false
+		return
 	}
 
 	bo := hostByteOrder()
@@ -52,4 +54,6 @@ func init() {
 		}
 	}
 	doinit()
+
+	Initialized = true
 }