ck: Correct asm output operand widths in amd64 pointer intrinsics

This does not appear to change generated code with the default
toolchain. However, KMSAN makes use of output operand specifications to
instrument inline asm, and with incorrect specifications we get false
positives in code that uses the CK_(S)LIST macros.

This was submitted upstream:
https://github.com/concurrencykit/ck/pull/175

The commit applies the same change locally to make KMSAN usable until
something equivalent is merged upstream.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation