Additional reader for slot containing the methods of a GF.

During phase 3 of bootstrapping, the generic function
GENERIC-FUNCTION-METHODS is used to compute wich methods are
applicable on a bridge generic function, so that the entire mechanism
for executing generic functions can work.  However, at the same time,
methods are added to target generic functions, so there must be a way
to access the methods of a target generic function as well.  It might
have been possible to convince the bridge generic function to do both,
but we didn't do that.  Instead, we have a second reader for the slot
containing the methods of a generic function which is used for target
generic functions.