cmd/gomobile: Fix issue in init when downloading full NDK.

Fixes an issue where the full NDK could not be downloaded due to a missing execute permission.

Fixes golang/go#12208

Change-Id: I6f432b5021b082693a02a293f50caf6da4d4e379
Reviewed-on: https://go-review.googlesource.com/13669
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/cmd/gomobile/init.go b/cmd/gomobile/init.go
index b03732f..137ee36 100644
--- a/cmd/gomobile/init.go
+++ b/cmd/gomobile/init.go
@@ -435,6 +435,10 @@
 	// is not used, and 7z.exe is not a normal dependency.
 	var inflate *exec.Cmd
 	if goos != "windows" {
+		// The downloaded archive is executed on linux and os x to unarchive.
+		// To do this execute permissions are needed.
+		os.Chmod(archive, 0755)
+
 		inflate = exec.Command(archive)
 	} else {
 		inflate = exec.Command("7z.exe", "x", archive)