Bug 971897: Use xpct{stubs,invoke}_x86_64 from linux/unix on OpenBSD/amd64 for consistency r=bsmedberg

This fixes the download panel issue, and brings us revs 727721e5d8ac,
844b142d8111 and 236989b3a807 as a bonus. Remove the unmaintained
OpenBSD/amd64 specific versions.