[IRGen] Don't use GOTPCREL relocations for x86 ELF.
Unforunately, x86 ELF linkers like to optimize GOTPCREL relocations by
replacing `mov` instructions that go via the GOT with `lea` instructions
that do not.
That would be fine, but they aren't very selective and will happily
perform this transformation in non-code sections if they think that
the bytes before a relocation look like a `mov` instruction.
This corrupts our metadata.
rdar://148168098
