[Orc] Enable debug object tests only on x86_64 hosts

These tests rely on running IR code with an explicit x86_64 target triple. They won't work on other architectures. (They won't work for 32-bit processes on x86_64 hosts either. We will take care of this later.)

Differential Revision: https://reviews.llvm.org/D107640

(cherry picked from commit c5ab55f5)