Enable IRGen/protocol_metadata test on Apple Silicon

[karwa]

This requirement seems to be too restrictive. The test works for me as-is on an M2 Max (removing the REQUIRES line, of course).

I assume the intention is to limit this to 64-bit architectures, and I think PTRSIZE=64 is the right way to do that?

[karwa]

@rjmccall You are listed as the code-owner for IRGen. Could you take a look?

[karwa]

ping @rjmccall

If you can't take a look yourself, could you please suggest somebody to review?

[rjmccall]

This looks reasonable to me.

[rjmccall]

@swift-ci Please test