This adds an independent mark phase to the GC that can be used to
verify the the default concurrent mark phase has found all reachable
objects. It uses the upper 2 bits of the boundary nibble to encode
the mark leaving the lower bits to encode the boundary and the
normal mark bit.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/167130043