app: support C compiler that defaults to -fno-common

For the Android NDK, between r21 and r22 the NDK compiler switched the
default from "-fcommon" to "-fno-common", which causes duplicate
definitions when building the examples given the current C code. To
fix things, rework the C code to insure that we have a single
definition and an "extern" reference to the objects in question.

Updates golang/go#44121.

Change-Id: If198d5cfd977a7cfd1a37652d3a1916d8c8dbcb7
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/290031
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Trust: Than McIntosh <thanm@google.com>

Author: thanm

app/android.go

@@ -35,8 +35,9 @@ package app
 #include <pthread.h>
 #include <stdlib.h>
 
-EGLDisplay display;
-EGLSurface surface;
+extern EGLDisplay display;
+extern EGLSurface surface;
+
 
 char* createEGLSurface(ANativeWindow* window);
 char* destroyEGLSurface();