app: support C compiler that defaults to -fno-common
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 <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Trust: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/mobile,golang/mobile,golang/mobile,golang/mobile