tests/short-first-segment: fix uname(1) usage and skip message

[klemensn]

Replace non-portable GNU -i, --hardware-platform with -m[, --machine] to fix "unknown error" usage error on, e.g. OpenBSD.

Also fix the check's backwards logic. OpenBSD/spar64 now prints -skipping test: not supported on x86_64 Linux +skipping test: amd64 Linux required