[MC] set OpenBSD's ELFOSABI by default (#98158)
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.
