os: Executable can use /proc/self/exe on android

Fixes the os test on the Android builder.

Change-Id: Ibb9db712156a620fcccf515e035475c5e2f535a5
Reviewed-on: https://go-review.googlesource.com/33650
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/executable_procfs.go

@@ -19,7 +19,7 @@ var executablePath, executablePathErr = func() (string, error) {
 	switch runtime.GOOS {
 	default:
 		return "", errors.New("Executable not implemented for " + runtime.GOOS)
-	case "linux":
+	case "linux", "android":
 		procfn = "/proc/self/exe"
 	case "netbsd":
 		procfn = "/proc/curproc/exe"