cmd/gc: don't generate algs for internal map types.

Fake types describing the internal structure of hashmaps are
generated for use by precise GC.

Generating hash and eq functions for these fake types slows down
the build and wastes space: the go tool binary size is 13MB
instead of 12MB, and the package size on amd64 is 48.7MB instead
of 45.3MB.

R=golang-dev, daniel.morsing, r, khr, rsc, iant
CC=golang-dev
https://codereview.appspot.com/13698043