runtime: fix scanning of gc work buffer

GC types were not being generated for the garbage collector
work buffer.  The markfor object was being collected as a result.
This broke amd64p32 and maybe plan9 builds.  Why it didn't break
every build I'm not sure...

Fixes issue 8812

LGTM=0intro, rsc
R=golang-codereviews, dave, khr, 0intro, rsc
CC=golang-codereviews
https://codereview.appspot.com/149260043