Make the native library on android also a valid shell executable

On android, executables are PIE, which mean that we can have a native library
that is also an executable. Use this to have the native library of the shell
be a valid shell executable.

Also, allow the shell to find it when it wants to fork and exec itself, as a
PoC.

R=ppi@chromium.org

Review URL: https://codereview.chromium.org/756423004