Tests: Do not use i386 architecture with Xcode 10 and above

Xcode 10 dropped support for i386 builds.
9 jobs for 18099-legacy-xcode-build-system
Status Name Job ID Coverage
  External
passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager