From commit cf6e6f14d3, (wrappers/uname: report correct machine type
on ethereal, 2020-02-10).
Some template use `env -i` to reset environment variable for building
host's binary file, thus the wrapped `uname -m` reports empty strings.
Fix it by falling back to the value reported by `/usr/bin/uname -m`.
We don't expect people cross-compile from i686-ethereal-chroot on top
of x86_64 machine, it was expected to use for travis to native compile
i686 only.
447bae0d3e