Don't run a.out if we might be cross compiling.

$cross_compiling might be set to "maybe" when we're building for x86
Android. There seems to either be a race in fgets or an
incompatibility of running bionic on the host (which we depend on for
the bionic host tests) that causes this absolutely trivial program to
hang. As a work around, just admit that we actually are cross
compiling.

Bug: http://b/24016357
Change-Id: I054a19c9516a6ed972d47aa80000c357b64da804