cmd/gomobile: put libopenal.so to lib/armeabi-v7a directory
cmd/gomobile: put libopenal.so to lib/armeabi-v7a directory

where compiled go shared library is located. Android expects
all shared libraries to be in the same directory.

Missing from https://go-review.googlesource.com/17860

Fixes golang/go#13911

Change-Id: I832c6571c2655d4114e0a0c93e663a6b4be702e3
Reviewed-on: https://go-review.googlesource.com/18820
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

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