fix issue #486 (Release MemObject on multi-devices platform)
fix issue #486 (Release MemObject on multi-devices platform)

When we create a buffer on a multi-devices platform, we do not want
any clRetainMemObject on it so that only one clReleaseMemObject is
needed to free the buffer.

Oblomov/pocl,franz/pocl,franz/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,pocl/pocl,pocl/pocl,Oblomov/pocl,rjodin/pocl,franz/pocl,rjodin/pocl,pocl/pocl,rjodin/pocl,rjodin/pocl,Oblomov/pocl