Turns out that `adr` doesn't work in inline assembly within LLVM on
arm macOS, or at least not how we were using it. This switches instead
to an `adrp` and `add` pair which seems to convince the linker that the
relocations should all fit. The same pattern is used on Linux as well
only it has different syntax (so much for a portable assembler) for
consistency. Performance isn't really an issue here so there's no need
to go out of our way to get the single-instruction operand working.
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