Detected when cross building timg after rebuilding GraphicsMagick with
new libdir directories. Could be fixed in all offending packages, but
it's easier to fix it in one place, and arguably more correct.
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.
We're using Ethereal chroot style on Travis CI.
In that chroot style, `uname -m` reports x86_64 even if we're
bootstrapped with i686 systems.
Some i686 build that employ `uname -m` is failing on Travis CI,
e.g: https://travis-ci.org/void-linux/void-packages/jobs/648010517#L5787
Change `uname` output to make Travis Output meaningful for i686.
