[pauthabi64] Require 0 addend for signing of undefined weak ref

[smithp35]

Clarify that undefined weak-references are unsigned.

The intent was that null-pointers could be easily tested for without worrying about signing schemas, if there is an unresolved weak-reference with a non-zero addend then the resulting value doesn't make a lot of sense, signed or unsigned. In practice to use a weak-reference with an addend it must be tested first with a zero addend.

if (weak) {
use weak->addend;
}

Raised from: llvm/llvm-project#173296

[jrtc27]

Unless there's something else in the spec saying what to do for undefined symbols, I think this would imply that if the addend is non-zero you should sign the pointer as if the symbol were defined? Perhaps something more like this?

Suggested change

	For a relocation that involves signing a pointer, and the relocation
	addend is 0, if the target symbol is an undefined weak reference, the
	result of the relocation is 0 (nullptr) regardless of the signing
	schema.
	For a relocation that involves signing a pointer, if the target symbol is an undefined weak reference, the
	result of the relocation is not signed. If the relocation addend is 0, this means the result of the relocation is 0 (nullptr) regardless of the signing schema.

[smithp35]

Thanks for the suggestion, looks like I misinterpreted the original issue when coming back to it. I've adopted the suggestion and will update the title and description to match.