Hi guix,

Disclaim procedures do not seem to be used in the wild,
but seem useful for implementing ephemerons in Guile
(to be investigated), hence this patch.

Guile seems to start building fine when using a bdw-gc packave
with "--enable-disclaim" added (I'm using (package (inherit ...) ...),
but that was on master. I'll verify on core-updates, but building
things will take some time.

Greetings,
Maxime.