NetBSD has long since migrated to the EABI and doesn't officially support the
OABI anymore. The ABI selection logic in LLVM only actually picks OABI for
NetBSD as a last resort if the EABI isn't selected. That fallback is likely to
be removed in the future. So just remove this support in Zig entirely.
While here, I also removed some leftover 32-bit Arm and 32-bit x86 code for
Apple targets, which are long dead and unsupported by Zig.
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