runtime: try to determine the actual type during garbage collection
runtime: try to determine the actual type during garbage collection

If the scanned block has no typeinfo the garbage collector will attempt
to get the actual type of the block.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/7093045

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme