This matches what is done for FreeBSD.
OpenBSD has a few special program header types, and other such ELF
extensions. Setting the ELFOSABI like so will allow LLD to support them
without needlessly impacting non-OpenBSD ELFs.
Testing strategy matches 06cecdc60ec9ebfdd4d8cdb2586d201272bdf6bd.
Take two of #98158 / b64c1de714c50bec7493530446ebf5e540d5f96a, which was
reverted in #98494 / c0261351136e4a826be697e5ebb5fa638abe7485.
Preexisting test is fixed now.